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Full text of "The optical lantern, for instruction and amusement"

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Price, One Dollar. 




II SCOVILL & ADAMS COMPANY, 

423 Broome Street, New Y^~k. 




The Author's Apparatus. 






THE 



Optical Lantern, 



FOR 



Instruction and Amusement. 



BY 



/ 
ANDREW PRINGLE, F.R.M.S., 

President Photographic Convention of the United Kingdom, i88q, 

Author of "Practical Photo-Micrography" "Lantern Slides 

by Photographic Methods" and Joint-author of 

"Processes of Pure Photography." 



4 i ^ 






NEW YORK; 

THE SCOVILL & ADAMS COMPANY, 

423 BROOME STREET. 

1890. 




Copyright, 1890, 
The Scovill & Adams Company. 



na 



PREFACE. 



It may fairly be said that this book fills a place not occupied 
by any other at present before the public. For the present 
treatise deals with the Optical Lantern only ; it does so, I trust, 
as fully as is required for any purpose ; no attempt is made as 
in other books on the " lantern " to give instructions for lantern- 
slide making, still less for the production of negatives. The 
present is in fact a lantern book, and a lantern book only. As 
such I hope it will be useful. 

I have written less for the popular public lecturer than for the 
photographer and the teacher. If in any degree my writings 
popularise and simplify the use of the optical lantern, specially 
if they tend to give the lantern a locus standi as a permanent 
part of the paraphernalia of the lecture room, I shall be happy 
in the consciousness of having done some good. 

ANDREW PRINGLE. 



CONTENTS. 



Introductory, 
General System, 
The Condenser, 

The Projecting Lens, 
The Lantern Body, 



CHAPTER I. 



CHAPTER II. 



CHAPTER III. 



CHAPTER IV. 



CHAPTER V. 



PAGE 

7 

10 
13 

19 
23 



CHAPTER VI. 

Double and Triple Lanterns, Dissolving Views, "Effects," Etc. 29 

CHAPTER VII. 
The Lime Light, (1) The Jets, ----- 35 

CHAPTER VIII. 

Jets Continued, Details and Accessories, 44 

CHAPTER IX. 

Preparation of Oxygen and Hydrogen, 47 

CHAPTER X. 

Storage of Gases (1), In Bags, Pressure Boards, Etc., ,- 54 

CHAPTER XI. 

Storage of Gases (2), Cylinders, Regulators, Gauges, - 59 

CHAPTER XII. 
Screens and Frames, ------ 64 



6 CONTENTS. 

CHAPTER XIII. 
Supports for Lantern, Slide Carriers, Reading Lamps, Etc., 69 

CHAPTER XIV. 
Lanterns and Apparatus for Experiments, 74 

CHAPTER XV. 

Practical Working of the Lantern, Oil Lamps, Blow-through 

Jets, -------- 80 

CHAPTER XVI. 

Practice with Mixing Jets, ----- 84 

CHAPTER XVII. 

Preparations for a Lecture, 87 

CHAPTER XVIII. 
Management of Lantern During a Lecture, 91 

CHAPTER XIX. 

Deportment on the Platform, ----- 94 

CHAPTER XX. 

Arrangements for the Lantern in a Lecture-room, - 98 

CHAPTER XXI. 

Enlarging with the Optical Lantern, ... 101 

Table of Enlargement, Etc., ----- 105 

Memoranda for a Lantern Display, - 106 



THE OPTICAL LANTERN 



CHAPTER I. 
INTRODUCTORY. 



When we consider that in a coat pocket one can with ease 
carry a dozen lantern slides, and when we realize that each 
of these slides can be projected as a picture of (say) twenty 
square feet in size by means of an optical lantern, the utility 
and importance of this scientific optical appliance must be 
very evident. We should also consider the ease with which 
these lantern slides may be made, either by photography or by 
hand ; we have also to take note of the variety of subjects 
amenable to reproduction as lantern slides, and we should 
further note that we are not even tied down to lantern slides 
as objects for the lantern, but that we may by our lantern 
project enlarged images of opaque objects, chemical and 
physical experiments and microscopic objects. When, finally, 
we find that all this is accomplished by an instrument which 
can be purchased by a moderate purse and carried by the 
feeblest hand, we are bound to own the merits of such an 
instrument, more especially because we know that what it 
pretends to do it does well. 

No defence of, nor plea for, the optical lantern is needed in 
addressing circles or societies addicted to photography, for 



8 THE OPTICAL LANTERN. 

there are but few photographic memberhoods that have not 
their optical lantern, or at least their occasional lantern display. 
But most singular is the slowness of the progress in lantern 
use among societies and in educational institutions not purely 
photographic. True, the chief of our learned societies now- 
adays have their optical lanterns however unworthy the instru- 
ment may in some cases be ; but there is in no educational 
institution, so far as the writer knows, a regular system of 
teaching or illustration by lantern projections. From a purely 
ocular point of view, diagrams, maps, blackboard designs and 
the like are inferior to the much larger and clearer lantern 
illustrations, and the shortsighted student must miss much of 
what the more fortunate normal-eyed may catch. But taking 
higher ground, it may be asserted that where it is important 
to study things as they really are, and not as the teacher is 
able to draw them, or thinks they are, or wishes them to 
appear, the optical lantern, with or without photography, 
must have the strongest claims on our attention. However 
well a physiological or pathological object may be drawn by 
hand, such a drawing is impotent to carry conviction and to 
impress memory in comparison with an enlarged photograph 
or an enlarged image of the object itself. When was ever a 
spectrum, drawn by hand and colored by any method, able to 
compare with the actual spectrum projected on a screen by 
means, for instance, of an electric arc ? 

Many professors have admitted to the writer the claims of 
the optical lantern as a useful scientific appliance ; some have 
owned that photographic lantern slides would be far superior 
to any other known method for educational demonstration ; 
but each and all of these able men have been deterred from 
inaugurating the system we recommended by the most extra- 
ordinary and imaginary misgivings as to the cost of install- 
ment, and the difficulty and even danger of working the 
apparatus. The difficulty, strange to say, most commonly 
foreseen, was that of darkening the lecture-room — with a 
touch of doubt as to the conduct of young students in a dark- 
ened room, and there was the usual dubiety as to the safety of 
the gases used for the lime light. 



THE OPTICAL LANTEKN. V 

The prime object of this book is to remove dangers and 
difficulties from the imagination (for there alone they exist) 
of many who are prepared to admit the advantages, but who 
question the practicability, of using the optical lantern as an 
educational and recreative instrument. 

There are many persons, especially photographers and 
public lecturers, as well as philanthropists striving for the 
good of their fellow-men, who use the optical lantern, but they 
use instruments of such poor quality, and appliances so nearly 
obsolete, and they so little understand the means for getting 
the best results from the appliances they have, that the writer 
feels it no impertinence on his part to try to put "lantern 
matters" before the public in a succinct, and, as he hopes, 
clear and intelligible manner. He will endeavor to cover all 
essential ground, to reject all side issues, and to deliver him- 
self in terms precluding possibility of mistake. 




CHAPTEK II. 



GENERAL SYSTEM. 



The essentials of the optical lantern as a means for the 
projection of enlarged images from smaller objects on a 
suitable surface are very simple. 

The optical system consists essentially of (1) a radiant ; (2) 
a condensing system for collecting and concentrating on the 
object the light rays furnished by the radiant ; (3) a projecting 
system for collecting, projecting, enlarging and focusing the 
image of the object as illuminated by the rays from the con- 
denser ; (4) a " screen " or surface for receiving the enlarged 
image and making it visible to the eye. 

The entire system will be easily understood from Fig. 1. 
In this figure enough is drawn to show the general optical 
principles iuvolved, the projection lens is made with an unusu- 
ally large back combination not necessary with symmetrical 
combination lenses. 




Fig. 1. 

R, the radiant ; C, the condenser ; S, the lantern slide ; P, the projection 

lens ; I, the screen or plane where the enlarged image is received. 

We may begin by describing what in most cases is the 
object to be illuminated and to be reproduced on a scale so 
much larger and at some considerable distance from the site 
occupied by the object. In the vast majority of cases this 
object is a transparent positive picture on glass, and the size 



THE OPTICAL LANTERN. 11 

of the picture actually to be projected is usually about three 
inches, as the diameter of a circle, or three and three-quarter 
inches, as the diagonal of an oblong. For convenience the 
glass plate is usually made three and a quarter inches square, 
an opaque mask having an aperture circular or oblong being 
used as a protection to the film of the " slide," and as a preven- 
tive of unsightly and superfluous light on the screen. The 
"slide" may be a " lantern slide " produced by photography, 
or it may be a hand-sketched design ; for the present we will 
assume it to be a " transparent positive " on glass three and a 
quarter inches square, with a mask having an aperture circular 
or oblong of the above dimensions. 

The radiant may be one of several kinds, but our chief con- 
sideration is that it may be a point of light, approximately, as- 
an incandescent point on a lime cylinder ; or it may have a 
surface of incandescence or be formed of several incandescent 
surfaces as in a multiple-wicked lamp. The more nearly the 
radiant approaches to being a point of incandescence the 
better, provided that the incandescence is sufficiently brilliant. 
The point of incandescence must be precisely in the focus of 
the condenser if the best result is to be obtained ; enlarging 
the area of incandescence is a makeshift and often a very bad 
one. Still, as oil lamps are used, and are often convenient, 
we must consider wicks as our radiant as well as limes or arcs. 

The condenser collects the light rays from the radiant, and 
compresses or condenses them on the slide which should be as 
close as convenient to the condenser. The rays pass from the 
condenser through the slide to the front conjugate focus of 
the condenser, and at or near that front focus the projection 
lens takes up the image of the slide, and projects it for- 
ward to the screen which is usually white and opaque, but 
is sometimes translucent. The sharpness of the picture on the 
screen depends upon the slide and the screen being in the 
planes of the posterior and anterior conjugate foci of the 
projection lens; and as the screen once erected is to be 
considered immovable, while the position of the projection 
lens varies with its focal length in its relation to the slide, the 
lens requires to have a certain amount of range of focus, that 



12 THE OPTICAL LANTERN. 

is to say, must to a certain extent be adjustable as regards 
its position with respect to the slide. More technically, 
the projection lens should have a rack and pinion focusing 
movement. 

An optical lantern is simply a device for holding the radiant, 
the condenser, the slide, and the projection lens in suitable 
positions with regard to each other ; and that the radiant may 
not directly illuminate the screen, nor interfere with the view 
of the spectators, the lantern usually takes the form of an 
oblong box, the light and part of the condenser being inside the 
box, the slide-carrier and projection lens attached to the front. 
A lantern box ought to be fairly light tight ; and it ought to 
be as small as possible consistently with giving room for the 
radiant and not itself becoming too hot. In our opinion 
optical lanterns are made ridiculously large, and we hope to 
see in the future our lanterns for lime-light much more conven- 
ient and less weighty and expensive. With oil lamps we have 
to deal with the very important matter of draught or ventila- 
tion, which must govern the size of the lantern box to a 
considerable extent. 

The slide is usually held in the lantern, or passed through 
it, by means of what is called a "carrier"; the screen is often 
sustained by a " frame " or suspended from a roller ; these 
details are not essential to the system and will be treated later, 
each in its proper place. 



CHAPTER III. 



THE CONDENSER. 



The condenser is used to collect rays of light that would 
without it be lost so far as illumination of the slide is 
concerned. The radiant is stationed at one conjugate focus 
of the condenser, while the projecting lens is or ought to be 
somewhere near the other conjugate focus of the condenser ; 
and the pencils of light travelling from the condenser in 
convergent lines towards the front focus ought to illuminate 
the entire aperture of the slide. The first point we have to 
consider with regard to a condenser is its area, or more 
conveniently, its diameter. A large condenser will illuminate 
a larger area of slide than a small condenser. But a large 
condenser requires to have a long focus practically, so that we 
lose light, as will be shown later. And in the matter of focal 
length of condensers, we have always to remember that there 
is a limit of shortness of that focal length, for such a radiant 
as incandescent lime, not to mention an electric arc, cannot, 
without grave danger to the condenser, be brought nearer 
than, say, two and a half inches from it. 

In practice, then, we have to choose between condensers 
having diameters of three and a half, four, and four and a 
half inches. If the reader will examine our figure, he will see 
that in order to illuminate the whole surface of a slide the 
condenser must have an area at the very least equal to the 
surface area of the slide. If now our slide has a mask with 
round aperture three inches in diameter, and if the slide is 
pretty close to a three and a half-inch condenser, the whole of 



14 



THE OPTICAL LANTERN. 



the aperture in the slide will be illuminated by the condenser, 
as in Fig. 2. 

C S 



R 

K 





Fig. 2. 



If in fact, the whole of a three-inch condenser were effective, 
and if the slide could be placed close up against it, a three- 
inch condenser would suffice to illuminate a three-inch 
aperture slide. But in practice the entire surface of a 
condenser cannot be made available, and we cannot as a rule 



R 


r 
/ 
/ 
/ 
/ 
* 

/ 


\/ 




""* - . 




D ; 


\ 
\ 

V 

\ 

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___„ 


11 




Fig. 3. 

place the slide close up against the condenser, and so we may 
take it that a three and a half-inch condenser is the smallest 
practically available for a three-inch aperture slide. In Fig. 
3, we show the effect of an exaggerated distance between 
condenser and slide, A being a longitudinal and B a vertical 
section on the optical axis of the system. But suppose we 





Fig. 4. 



Fig. 5. 



have or intend to use an aperture say of the shape known as 
" cushion shape." A reduced plan of such a mask is given in 
Fig. 4, and we have placed inside it a three and a half -inch 



THE OPTICAL LANTERN. 15 

condenser on the same scale but not quite touching the slide. 
Plainly, taking the condenser of three and a half inches at its 

best, it cannot illuminate the cushion which in actual meas- 
urement has diagonals of three and three-quarter inches. It 
will, however, be found that a condenser of four inches 
diameter will cover the ordinary cushion shape aperture, 
or the dome, Fig. 5, and so we may accept four inches as 
sufficient diameter for a condenser for any picture on a three 
and a quarter inch slide, provided that the slide be placed 
near enough to the condenser. 

Some people seem to think that they will improve matters 
and make quite sure by using a four and one-half -inch condenser. 
There are slides made, or there were at one time slides made, 
of the same size as a "quarter plate," i. e. 4^x3J inches. 
These with their masks required a four and one-half-inch 
condenser sometimes, but we believe this size of slide is now 
obsolete, certainly it has no advantages over the commoner 
size, three and one-quarter inches square. There is a positive 
disadvantage in a four and one-half -inch condenser which we 
must point out. A four and one-half -inch condenser must almost 
necessarily have a longer focal length than a four-inch if 
aberrations and absorptions of light are to be avoided, and 
aberrations in a condenser are most serious defects. The focal 
length of a condenser may be taken for our present purpose as 
the distance between the radiant and the condenser. 




Fig. 6. 

In figure G, A is the radiant, B C the condenser, and A D the 
focal length of B C. With a three and one-half-inch condenser 
of any ordinary type the distance A D may be about two and 
one-half inches, with a four inch about two and three-quarter 
inches, with a four and one-half -inch it will be about three and 
one-quarter inches or more. Now according to the well known 



16 THE OPTIC A.L LANTERN. 

law the intensities of the light acting at D vary inversely as the 
squares of these distances, that is to say the four-inch condenser 
has the advantage over the four and one-half-inch one, in approx- 
imately the proportion of four to three. We cannot with safety 
shorten the focus of our condensers beyond the limits named, 
two and one-half inches, because with lime or electric light 
there would be the great danger of cracking the back 
combination of our condenser by the heat of the radiant. So 
to sum up, we may say that a four-inch condenser is on the 
whole the best for general use ; if only round apertures are to 
be used in the slides a three and one-half -inch condenser will 
be better ; if larger slides must be used the condenser must be 
larger. (See Chap. XXI). 

We now turn to the construction of the condenser. The 
simplest form is known as a bull's eye, and consists of a single 
piece of glass plano-convex in form. Practically it is useless for 
our purpose, it is not achromatic, has gross aberrations, and is 
curvilinear. 

The construction usually found now-a-days is that of two 
plano-convex glasses, placed with their convex sides nearly in 
contact, as in figure 1, C. The pair of glasses are mounted in 
a brass cell which fits in the front of the lantern, and it is 
important that there should in the circumference of the cell be 
air holes, so that when the condenser gets heated the air may 
escape from between the glasses, and " sweating" be thereby 
avoided. ' The glasses, moreover, should not be too tightly 
mounted in the cell, so that on being heated they may have 
room for moderate expansion. 

In the Almanac of the " British Journal of Photography " 
1888, we find Mr. J. Traill Taylor, a prime authority, 
recommending a form of condenser which we believe to be 
older than the one last described by us. This form consists, 
as will be seen from figure 7, of a plano-convex or slightly 
meniscated lens in close proximity to a double convex. Mr. 
Taylor then rightly points out the loss of light certain to arise 
with this form of condenser unless the focus be long, which 
will entail loss of intensity, and he proceeds with his usual 
ingenuity to suggest the interposition of a third lens of piano- 



THE OPTICAL LANTERN. 



17 



convex or menscus form between the light and the doublet 
combination previously representing the entire condenser. 
The figures are Mr. Taylor's, and almost explain themselves. 

/Cb 



<g — 
A>s> 




\l 




Fig. 7. 



Fig. 8. 



In A, a, b, represent rays that are lost, while in B, (fig. 8) 
they are refracted and utilized by the addition of the third 
and smallest lens. Mr. Taylor therefore concludes in favor of 
triple condensers. 

Our experience is confined to the form first figured (fig. 7) 
and the part of Mr. Taylor's figure A, of these we prefer the 
two plano-convex glasses, (fig. 1.). 

A very good double condenser is that of the late Mr. J. H. 
Dallmeyer, who described it to the Photographic Society of 




Fig. 9. 



Great Britain in April, 1880. As it is of somewhat curious 
form we give a diagram of it. 



18 THE OPTICAL LANTERN. 

The effective diameter is four inches, the equivalent focal 
length two and three-quarter inches. The lens A. is a flint 
plano-convex, three and five-eighth inches diameter, while B 
is a crown glass double convex four inches diameter. A. got-s 
next the radiant. 

Mr. Dallmeyer, in the paper wherein he described this con- 
denser, dwelt upon certain points which we cannot do better 
than emphasize here. A condenser, especially the lens next 
to the slide, must be of the most perfect glass, white, limpid, 
and free from bubbles, streaks, striae and scratches. Any 
mark, on the front element in particular, will show on the 
screen to the great detriment of the picture. The greatest 
care must be taken to preserve the condenser from ill-usage 
and noxious vapors. 

No condenser that ever was made will work well if the 
radiant be not centred with the condenser and in the focal 
point of the condenser. If the radiant in a lantern be out of 
centre the image on the screen will be unevenly lighted ; if 
the radiant be behind or in front of the condenser's focus there 
will be abominable fringes of color round the margins of the 
disc on the screen. In oil lanterns the light is usually centrally 
placed in the optical axis, and the lamp has to be pushed and 
pulled till the focus of the condenser is found — as nearly as it 
may be found with a multiple-wicked lamp. 

Note. — Frequently, when a projection lens of long focus is used — say, 
8 inches or over — a condenser of longer focus will be required, which, as 
a rule, implies a condenser of larger diameter. Thus with front lenses of 
8 inches or over we generally require to use our 4^-inch condenser The 
reason of this will be evident to any reader who will trace the course of 
rays from an ordinary 4-inch condenser ; he will find the rays cross before 
they reach the projection lens of long focal length. 



CHAPTER IY, 
THE PROJECTING LENS. 

The "Projecting," or as it is often loosely called, the 
" Front " lens is in certain respects of no less importance than 
the condenser. It so happens that ordinary photographic 
lenses are in the main well adapted for use as projecting lenses 
for the lantern, and so as a rule there is little fault to find 
with the lenses used by one who is a photographer or by a 
photographic society. But in the hands of the general public 
we often find projection lenses totally unsuited for their work. 

A projection lens requires in particular three qualities. It 
must be accurately corrected for the visual rays of the spec- 
trum ; it must have a wide working aperture, and it , must 
be free from spherical aberration. On account of the correc- 
tions of a photographic lens wherein the visual and chem- 
ical rays are made to coincide in focal point, such a lens as we 
have said is usually well corrected for the visual rays, but 
when a lens is only corrected for visual rays, and the actinic 
rays unheeded, that lens may be a splendid lantern lens, but 
cannot without modification be used as a photographic lens. 
A photographic lens of the type used for portraiture, in par- 
ticular one of the rapid lenses invariably used in the days of wet 
collodion, meets practically every want of a projection lens. 
A portrait lens has usually a flat field, and area of aperture 
large in proportion to its focal length, so that we get a good 
picture on our screen with brilliant illumination. Decidedly, 
but not necessarily much, behind a portrait lens for our pur- 
pose, comes a so-caEed " single " lens, such as is often used in 
photography for landscapes ; the chief drawback to this class 
of lens being that to secure a flat field we have to " stop down" 
the lens and so entail loss of light. Better perhaps than a por- 
trait lens is a lens made specially for the lantern by several 



20 



THE OPTICAL LANTERN. 



opticians, of whom we may mention Mr. Dallmeyer, of Lon- 
don, and Messrs. Taylor and Hobson, of Leicester. "We are 
enabled to give a diagram of a lens by the latter firm, we have 
worked one of eight inches focus made on these lines and it 
leaves nothing to be desired. 



1 




Fig. 10 (a). 



Fig. 10 (b). 



Some remarks as to the focal length of the projection lens 
seem to be called for. Given a certain distance from lens to 
screen the shorter the focns of the lens the larger will be the 
disc on the screen. Given a certain lens the nearer it is to 
the screen the smaller the disc on the screen. Given at a cer- 
tain distance from lens to screen a disc too large, we can make 
is smaller either by taking the lantern nearer to the screen, or 
by using a longer focus lens. A four inch lens will give at 
any distance a disc just twice the diameter of that given by an 
eight inch lens at the same distance. Mr. Chadwick gives a 
table that the writer has often found convenient. We quote 
it here : 

Let S= Aperture of slide-mask in inches. 

F=Foeal length of projection lens in inches. 
L=Distance from lens to screen in feet. 
D=Diameter of disc in feet. 



Then 



L= 



DXF 



~D 



LXS 



F= 



LXS 



S ^ F D 

Another useful guide to find the distance required from lens 
to screen for a certain disc is : calculate by how many diame- 
ters the disc is greater than the slide opening, add one to this 
number and multiply by the focal length of the lens. Thus : 
we require a 15 feet disc with a 6 inch lens ; 15 feet is 60 



THE OPTICAL LANTERN. 21 

times 3 inches, which we suppose to be our slide aperture ; 
61x6=366 inches=30 feet 6 inches, distance required from 
lens to screen. 

It is for various reasons usually a mistake to choose a lens 
of very short focal length. If the room is small there is not 
much choice, but in a larger hall we like to get as far back as 
we can up to 80 feet, at this distance a 12 inch lens gives a 
20 feet disc. But beyond this we do not go if we can help it. 
If the lantern has, on account of shortness of focal length of 
lens, to be placed, say, 25 feet from the screen in a moderate 
sized hall, many people behind the lantern will be prevented 
from seeing properly, and the apparatus will probably be sur- 
rounded by part of the audience, which is objectionable and even 
dangerous if bags are used. Moreover, if the screen is placed 
on a raised platform the lantern has to be canted up and the 
screen tilted forward to an inconvenient degree, whereas if the 
lantern were, say, 60 feet distant from the screen, the neces- 
sary cant and tilt would be much less, and the loss of light 
by greater distance is not nearly so great as some thoughtless 
persons seem to fancy, for the oft-repeated formula about the 
" intensity of light varying as the squares of the distances " 
does not hold here at all. The loss of light in enlarging a 
three inch disc to a fifteen feet disc is, cceteris paribus, practi- 
cally the same whether the enlargement be produced by a four 
inch lens or an eight inch. But if our projecting lens has a 
focal length either so short or so long that its back combination 
fails to grasp some of the rays proceeding from the condenser 
through the slide, then surely we shall lose light (see note 
to Chap. III.). By altering our condenser's focal length 
we may cram more pencils of light into a long focus front 
lens, but then we shall lose some of the pencils of light 
between the radiant and the condenser, or else have to 
remove the radiant further from the condenser, in which 
case our ancient formula will hold good as touching the 
illumination of the condenser itself. Reference to our Fig- 
ure 1, will show at once that if the front lens have a very 
short focal length it will not catch all the pencils of light pass- 
ing through the slide, unless it, the lens, has a very wide back 



22 THE OPTICAL LANTERN. 

combination, and again, if we get an aperture very wide in 
proportion to our focal length we shall have a lens impossible 
to correct for aberrations. So, as usual, it is a case of " give 
and take," we cannot have perfection in anything optical. 
Speaking from experience, we may say that from a quarter- 
plate portrait lens of about five and a quarter inches focal 
length to a lantern lens of eight inches, we have no trouble in 
using our four inch condenser ; with a ten or twelve inch lens 
we have had to use a four and a half -inch condenser ; while 
with a very short focus lens we happen to possess, our four 
inch condenser does not work very well. We are perhaps 
safe in saying that for all-round work a lens of about six inches 
focal length and a condenser of four inches as usually made 
will do as well as any other one-lens battery. The most useful 
lens we have for the lantern has a focal length of eight inches, 
at forty feet we get a grand fifteen feet disc, using a circular 
slide mask. 




CHAPTER V. 



THE LANTERN-BODY. 



As already stated, the functions of the lantern-body or 
box are simple, and consist merely of holding the parts of the 
optical system together and enclosing the illuminant so that 
damaging rays of light cannot reach the screen to spoil the 
image, nor the eyes of the onlookers to dazzle them. 

"When the radiant is an oil lamp the lantern body must be 
fairly large, for the lamp is sure to be of considerable size 
if a powerful one with say three wicks ; there must be a 
certain amount of unoccupied space inside the lantern to give 
draught and adjustment room for the lamp ; and, as a rule, 
the lamp requires a somewhat tall chimney to ensure draught. 
The body of the lantern is almost always " jacketed " wholly 




Fig. 11. 

or in great part, in order to meet the great heat given off by 
the lamp ; sometimes the outer jacket is of metal, sometimes 
it is of wood, the inner jacket being invariably metal. 

Perhaps the first really business-like lantern introduced 
with any success was the now well-known sciopticon of 
Mr. Marcy. In many points it formed a standard for later 
modifications. Fig. 11. 



24 



THE OPTICAL LANTERN. 



The double wick of the sciopticon lamp was soon found 
to be faulty in several ways, and a third wick was by various 
makers and in various forms added. Messrs. Newton greatly 
improved the performance of the lanterns of that day by 
placing a suitably curved reflector behind the wicks, their 
oil lamp being known as the "Kefulgent." In most oil 
lamps will be found in front of the wicks a piece of flat 
glass to prevent breakage of the condenser. In our expe- 
rience a well-arranged trio of wicks is better than two wicks 
only; we are informed that even five wicks have been 
successfully introduced. It must never, however, be forgotten 
that number of wicks does not necessarily mean increase of 
effective light, and so far as our knowledge goes we believe 
the best multiple-wick combination to be one of three wicks, 
placed longitudinally in the optical axis, the two outer wicks 
slightly inclined inward towards the centre one. The dangers 
with many-wicked lamps are : great heat, and a shadow down 
the centre of the image on the screen. 

Messrs. Laverne & Co., of Paris, who have vast experience 
in lantern making, turn out a serviceable lantern sold in 




Figs. 12 (a) and 12 (b). 
America as " The Scovill." It is made of Kussian iron, 



THE OPTICAL LANTERN. 



25 



jacketed, with a cool-air chamber round the condenser ; and 
can be had with three, four, or five wicks. Also often fitted 
to it is a front for showing opaque objects on the screen. 
Figs. 12 (a) and 12 (b) show this complete apparatus. 

But while a lantern-body for an oil lamp is necessarily of 
considerable size, we are convinced that lanterns for use with 
oxyhydrogen jets are usually made ridiculously large. A 
modern lantern, as usually made, is about three times as large 
an^. twice as heavy as it need be. With a lime jet there is 
no occasion for much unoccupied space, for no such draught 
is required as with an oil lamp, no chimney is required at all, 
and the only inconvenience to be feared is that of great heat. 
If the body be well jacketed, especially if the outer jacket be 
of wood, the total size of the lantern-body may be very greatly 
less than it usually is without affecting in any way the 
performance of the apparatus. Fig. 13 shows a lantern made 
by Messrs. Newton, of London, to the writer's order, and it 
embodies all the essentials of a perfect single lantern, though 
it is at least twice too large. 




Fig. 13. 



The receptacle at A, whereinto the "slide-carrier" goes, 
should have its front provided with a spring sufficiently 



26 THE OPTICAL LANTEKN. 

strong to hold the carrier in position while slides are being 
passed through. We propose to add to this part a couple of 
pinch screws, whereby the carrier once in position shall be 
clamped there. The space between the front of the condenser 
and the back of the "cone" or nozzle-tube to which the 
projection lens is screwed is sometimes required for a water- 
tank or other scientific article whose image requires to be 
projected upon the screen. An "open stage" is easily 
devised, or a lantern specially adapted for such demon- 
strations may be used (see Fig. 42, page 77). 

The projection lens is screwed or otherwise fixed to the 
"nozzle" of the lantern. This nozzle should "telescope" to 
a reasonable extent outwards, for on occasion a long-focus lens 
may be required, eight, ten, or even twelve inches focal length. 
It is very awkward to find that the nozzle will not stretch far 
enough for such a lens, when of necessity the lantern has to 
be a considerable distance from the screen. (If the lantern is 
to be used for " enlarging " purposes in photography there is 
all the more need for a long stretch of nozzle). 

Our next figure (No. 14) shows a lantern constructed for us 
by Messrs. Oakley, of London, and though much smaller than 
the lantern shown in fig. 13, it is precisely equal to the larger 
lantern in performance, the same condenser and lens being 
fitted to both. The dimensions of this miniature lantern are 
as follows : A to B six and one-half inches. C to A six and 
three-fourths inches. Width, five and one-half inches. 

The entire lantern-body is of iron, rivetted throughout. 
When packed the lens goes inside, while the back of the con- 
denser is protected by the tray, which keeps the lantern from 
falling forward when the apparatus is set up in use. The 
condenser is a four inch. 

Messrs. Newton also make a "miniature" lantern, and Mr. 
W. C. Hughes makes one with a cylindrical body. If the 
condenser were made square the size of the body might be 
still further reduced ; but as we can with ease carry our own 
lantern, packed with all its fittings, lenses, carrier and jet, in 
one hand, there seems to be no cause for further reduction of 
size. 



THE OPTICAL LANTERN. 



27 



Lanterns have always side doors, sometimes a door on one 
side only, sometimes a door on each side. The door is usually 
provided with a window of colored glass by which the worker 
is supposed to view the light without hurting his eyes. The 
writer has never yet been able to judge of the light through 
such a window, and has never felt any bad effect from looking 
directly at the light with half-closed eye-lids, a way by which 
he can judge of the quality of the light. It is certainly neces- 
sary to have at least one door which, however, during a display 




Fig. 14. 

should be very seldom opened. In every lantern the writer 
remembers to have seen there has been a considerable opening 
at the back of the lantern where the jet goes in, and lecturers 
of any experience must have noticed the mischief caused by 
this opening if any of the audience happen to be seated 
behind and near the lantern. A person so situated can see 
next to nothing on account of the glare from the lantern. If 
the lantern have a wooden outer body a curtain of thick velvet 
should be fixed to the top of the back of the body, and hang 
loosely down over this objectionable opening; our large 
lantern, figure 13, has at the back a door sliding in runners, 
and this door is pushed down after the jet is in position, but 
even this is not a sufficient protection in all cases. When the 



28 



THE OPTICAL LANTEEN. 



lantern body is of iron a similar piece of cloth may be hooked 
on to the lantern by metal hooks so bent as to make the cnrtain 
hang free of the hot metal body, if the body ever becomes hot 
enough to singe cloth, which it ought never to become if the 
lantern be properly arranged. 




CHAPTER VI. 

DOUBLE AND TRIPLE LANTERNS, DISSOLVING 
VIEWS, EFFECTS, ETC. 

We now come to those edifices known by various names 
and consisting of two or more lanterns made to work concen- 
trically, which appear to be as necessary to the professional 
itinerant lecturer as they are useless to the scientific demon- 
strator or the photographic amateur, except in a few very 
rare cases. The chief uses of these multiple-lanterns are : 
For what is called "dissolving" one picture into another, 
and for producing so-styled " effects " which are peculiarly 
the province of the popular public lecturer. Some of these 
" effects " are interesting, " effective," sensational, and really 
pleasing, and ought not to be despised ; we allude to changes 
from day to night, from one season to another ; while others 
on which great ingenuity in slide-making is spent, do not come 
within the province of this book nor touch those for whom 
this book is specially intended. There are some who think 
that in the display of a series of ordinary photographic lantern 
slides there is an advantage in " dissolving " from one view 
to the next ; that is to say, these parties prefer to see one 
image die gradually away and the next on the list grow grad- 
ually in a weird or " uncanny " sort of way out of the ghost 
of the last, rather than to have one picture follow another in 
the natural way. For our part we distinctly object to the 
dissolving business for. ordinary slides, but provided the 
masks of the slides be all of one size and shape, and the 
lanterns accurately registered, and the views or pictures 
reasonably adapted for such treatment, which they very 
seldom are, then by means of a double lantern the desideratum 
may be attained. And further, if it is deemed requisite or 
desirable to have the appearance of a curtain being drawn at 



30 THE OPTICAL LANTERN. 

the beginning of, and persisting throughout the lecture, it can 
be achieved by means of a third storey of lantern, the lantern 
being then a triple or more gloriously a " triunial," the top 
storey being sometimes lighted by oil. All these luxuries 
are obtained only at the expense of a doable or triple set of 
jets to look after, and to keep burning with equal brilliance — 
for if one disc is less bright than the other the effect is simply 
atrocious. If we are using, say, a double lantern, we have an 
arrangement usually very ingenious, often highly intricate, 
called a dissolver ; if we are using gas in all three lanterns 
of a triunial the dissolver becomes a matter, sometimes, for a 
life-study. Still we do not seek to discourage the triunialist, 
nor to underrate the value of good " effect " displays. 

In the early days of double lanterns the two lanterns were 
usually placed side by side, and the " dissolution " effected 
by serrated shutters in front of the projection lenses. We are 
indebted to Mr. George Smith, of the Sciopticon Co., London, 
for this detail sketch of the dissolver attached to his instru- 
ment. Of course there was a lamp in each lantern. Fig. 15. 









Fig. 15. 



The dissolver a is mounted on the arm I by means of a 
screw, as seen in Fig. 15, so as to cover alternately the lenses 
as it swings from side to side. The horizontal part of b slips 
into c till the length of the united axle just allows the dis- 
solver to swing clear of the lenses, and the whole is held in 
place by a socket spring at each end of the base-board. 

The dissolver is operated by the handles at c, which are 
adjusted at the proper angle to limit the lateral movement of 
a to the distance between the lenses. 

Now, however, nous avons change tout cela, and we place 
the lanterns one on top of another. There must be an 



THE OPTICAL LANTERN. 



31 



arrangement for causing the two discs to coincide precisely 
on the screen ; this, of course, is managed by giving facilities 
for tipping the upper lantern down or the lower lantern up, 
or both. Or the hinged parts may be entirely in front of the 
lantern-box proper, and the tipping may include only the 
front plate and all in front of it. The latter system is 
perhaps the most convenient. Double or biunial lanterns 
are generally made all in one piece, and the two lanterns 
cannot be separated, but it seems customary to make triples 










t 



Fig. 16. 



in separate parts — the top lantern almost always can be 
removed — so that if we have a triunial we have three separate 
lanterns. We figure a double and also a triple lantern which 
will stand each as a type of its own class ; so far as we nave 



32 



THE OPTICAL LANTERN. 



seen, the chief variations in these articles consist rather in the 
amount and finish of the brass-work, and in the price, than in 
any really essential qualities. 



MmM 




Fig. 17. 

From what has been said it will be readily believed that 
the arrangement for the distribution of the gases in suitable 
proportions to each jet, without loss of time or uncertainty 
from any one to any other of three lanterns, without waste of 
gas, must be a problem of considerable difficulty of solution. 
It is, however, excellently solved by more than one " dissolver." 
"We shall figure only two, and while Fig. 19 explains of itself 
the working of the instrument called the " One Plug," and 
made by Messrs. Otway & Son for triple lanterns only, the 
other gives a better idea of the general arrangement of a 
dissolver for a double lantern. 



THE OPTICAL LANTERN. 



33 




Fig. 18. 




Fig. 19. 



34: THE OPTICAL LANTEEN. 

In all cases, of course, there must be a " bye-pass" on the 
hydrogen ; in some there is a bye-pass on each of the gas 
tubes, with a pair of " blow-through " jets, for instance, it is 
necessary to have a bye-pass on the oxygen as well as the 
hydrogen. It will be noticed that the ■■ Star " dissolver, as it 
is called (Fig. 18), has a bye-pass (the small tap) on each side, 
as well as the long arm for the regular " dissolution." The 
bye-pass on the H. side has its evident use ; the O. bye-pass is 
requisite to prevent a " pop," or slight report, on dissolving 
after a prolonged time of projection from the other lantern. 
In testing a dissolver we have to note whether there is any 
noise on effecting the change, whether any time is lost before 
the second lime is fully heated, and whether any gas is wasted. 
A dissolver that requires only one hand to work it is naturally 
preferable to a less convenient one. For certain reasons we 
recommend that the long arm in Fig. 18 be prolonged to the 
same extent as shown, on the other side of the central boss. 




CHAPTEE VII. 



THE LIME LIGHT. 1. THE JETS. 

The radiant most commonly used, and as we think the best 
to use in the optical lantern, is known as the lime-light, or 
sometimes as the oxy-hydrogen lime-light. A point on the 
surface of a disc or cylinder of unslacked lime is rendered 
incandescent by the action of an oxy-hydrogen blow-pipe ; the 
lime is very " refractory," i. e. very difficult to cause to burn, 
but when it is rendered incandescent its incandescence is 
very brilliant. There are "soft" limes and "hard" limes, the 
soft ones being rendered incandescent more easily than the 
hard ones, and so the former are used when the heating power 
of the blow pipe is not very great, as for instance, with the 
so-called oxy-calcium light, or the " blow-through " or " safety " 
jet. But to get the best result possible with the lime-light, i. e. 
to get the greatest brilliance over the least area, we must use 
the hardest lime and a blow-pipe of proportionate heating 
power. 

The oxy-calcium light calls for little remark ; it is much less 
brilliant thaa the next higher light, the blow-through. In the 




BFH 



Fis. 20. 



oxy-calcium system we have a reservoir of spirits of wine ; a 
tube from reservoir to nozzle, where there is an ordisiarv wick. 



36 



THE OPTICAL LANTERN. 



The spirit is lighted at the wick in the usual way, and a stream 
or jet of oxygen gas is driven through the flame to the lime 
which is rendered incandescent, and gives a fairly bright light, 
estimated approximately at " 150 candles." 

We figure (No. 20) an oxy-calcium jet. 

The blow-through jet comes next in order of brilliance of 
light attainable. 




Fig. 21. 

The nozzle is made in slightly varying forms, but fig. 21 
shows a very good form, made by Mr. Hughes, of London. In 
the blow-through jet we use a fairly large column or stream of 
hydrogen gas, either direct from the usual house-supply or 
under a pressure low when compared with that of the mixing. 
jet. In or near the centre of this quietly burning stream of 
hydrogen is a small nipple, through which oxygen is forced at 




Fig. 21 (a). 



some pressure greater than that on the hydrogen ; the oxygen 
is blown through the hydrogen flame, whence the name given 
to this jet, which is also sometimes called the " safety." Fig. 
21 (a) shows a form of blow-through jet which gives as good 



THE OPTICAL LANTEKN. 



37 



a light as any form known to us. It is made by Messrs. 
Newton. The estimated power of a good blow-through jet is 
250 candles, but a disadvantage of this system is that the area 
of incandescence is generally large when the greatest total of 
light is given off, and the light is apt to be red in tint. 

The same gases are used in the mixing- jet, but both gases 
are usually under high pressure, and they are mixed in a 
chamber before they emerge from the nipple where they are 
lighted. The reason for the nomenclature is thus evident, and 
on the completeness of the mixture of the gases depends in a 
great measure the amount of heat, and consequently of light 
produced. Our fig. 22 shows a very good style of mixing-jet, 




Fig. 22. 



made by Mr. Place, of Birmingham, England, at a cheap rate. 
A. is the mixing chamber. 

In order that we may obtain the very best results certain 
principles must be attended to in the construction of all kinds 
of jets where the gases are under pressure, however low. 

In the gas-way there should be no sharp turn or acute angle 
corners ; even in the mixing chamber the " jostling " of the 
gases may be too violent. We do not ourselves recommend 
the putting of any obstacle whatever in the gas-way. Many 
jets by the best makers contain near the mixing chamber pieces 
of fine gauze. Whether this is intended to insure effective 
mixing of the gases, or whether it is put there with an idea 
of greater safety we cannot say, but we invariably remove and 
throw away the gauze. Some makers have themselves 
admitted to the writer that they knew of no advantage 



38 THE OPTICAL LANTEEN. 

belonging to the gauze, but that it was put there in deference 
to public opinion only. Many other substances have at one 
time or another been put in the gas-way — cotton, wool, shot, 
powdered pumice-stone, and even lengths of cane ; for what 
object unless to cause failure and danger we have never 
yet been able to ascertain. "With ether tanks and such 
arrangements there must be value in any device that will 
prevent a suck-back of gas, or will extinguish gas if sucked 
back lighted. We would not be misunderstood with regard 
to our position on the ether-oxygen question. There is no 
necessity whatever for accidents with the " eth-oxo " system ; 
accidents with it have occurred always through some mismanage- 
ment, or misunderstanding, or carelessness ; but. the fact remains 
that they have repeatedly occurred, even in experienced hands, 
and when they do occur the results are always alarming, often 
disastrous. Were there no system except the ether one, the 
writer's opinion might be modified, but as oxygen and hydrogen 
are easily made, if not easily procured, and as the common 
oxyhydrogen light is certainly equal to, probably superior to 
the ether-oxygen, the writer does not dare to recommend the 
only system that has, so far as he knows, led to any disastrous 
results, except such as could be accounted for by the very 
grossest carlessness or ignorance on the part of those who 




Fig. 23. 

came to grief in working the ordinary system. The writer is 
the more satisfied as to his wisdom in this matter from the fact 
that he is professedly writing for beginners in lantern matters. 
There should be no obstruction, then, in the gas-way, but if 
there is to be obstruction at all the gauze alluded to will do 
little harm, and a certain safety-valve may give confidence to 
a timorous worker. We show this valve, figure 23 ; by means 



THE OPTICAL LANTEKN. 



3'J 



of a simple internal arrangement gas is allowed to pass one way 
only. We believe Mr. W. I. Chad wick devised this. 

A very important point toward the production of a good 
result is the angle at which the gas-stream impinges on the 
lime. A simple figure (No. 24) will illustrate this. 



A B 

Fig. 24. 

A and B may be taken as extreme cases. If the angle of 
impact be such as suggested at A, the nipple of the jet would 
come between the point of radiance and the condenser, and in 
fact there would be a shadow of the nipple on the condenser. 
Moreover, the lime would be worn into a very deep " pit," 
which always hurts the light and endangers the condenser, 
because the little brilliant pit might have its heat-focus on the 
condenser, and even the flame itself might glance off the 
lime at such an angle as actually to strike the condenser. 



On the other hand, in B. the 



gas-stream 



impinges on the 



lime in such a slanting manner that the impact would be 
very imperfect, and there would be a want of " grip," so to 
speak. Mr. Lewis "Wright, a great authority in such matters, 
worked out the theory and practice of this subject, and as a 
result Messrs. Newton of London, England, produce a mixing- 
jet which is in the writer's opinion perfect, and has been in 
his practice invaluable (Fig. 25). 




Fig. 25. 
As this jet shows some novel features we shall have to recur 
to it. 



40 THE OPTICAL LAJSTTERN. 

The bore of both tubes and nipples varies in various makers' 
jets. It is impossible to lay down any laws on this matter. 
Perfection of light depends not on any absolute or isolated 
item, but on the proper adaptation of one part of the system 
to another. In particular, it may be said that the pressure of 
gases regulates all other conditions. A small bore tube may give 
a better light than a large bore, at a less expenditure of gas, if 
the pressure happens to suit the smaller tube. A large bore 
nipple works at a decided disadvantage if the pressure of gas be 
not up to a corresponding point. Doubtless the very best light 
is to be got from a large bore with a great pressure. Perhaps 
the greatest mistake made in ordinary practice is having a bore 
too large for the pressure. For general use the calibre of both 
tubes and nipple should be small rather than large. 

Every one who has worked with lime-light knows the 
unpleasant effect of "hissing," "buzzing," "whistling," or 
" roaring," by the jet. These noises may be due to various 
causes ; improper proportion of the gases to each other, im- 
proper position of the lime in relation to the nipple, pitting of 
the lime, etc. ; but the commonest cause is roughness in the 
tubes or nozzle, protrusion of shreds of metal into the gas-way, 
square corners in the gas-way, in fact, any rough edge or un- 
evenness inside the tubes. Sometimes the noise may be stopped 
by cleaning, or broaching, or riming out the tubes, but some- 
times jets are so badly made that noise is inevitable at any 
pressure over a very low one. Any jet may be noisy under 
certain conditions ; a jet should not, ipso facto, be rejected on 
account of noise. If a jet is a good one otherwise, but is noisy, 
it should be examined, cleaned out, rimed out, if necessary ; 
failing these remedies, if the noise is intolerable it may be 
rejected, but reduction of pressure is likely to obviate the noise. 

The "nipple" of the nozzle of a jet sometimes screws on to 
the nozzle, sometimes is in one piece with it ; sometimes is 
entirely of brass, sometimes has a platinum tip let into it. 
Some first-class lanternists find no advantage gained by the 
platinum tip, while others advocate it. If the tip is used, 
it must be properly adjusted to the brass, so as not to 
form a projection into the gas-way. Sometimes in very 



THE OPTICAL LANTERN. 41 

powerful jets the platinum-tipped nipples lead to trouble, on 
account of the platinum separating or burning out from the 
brass. In our opinion the bore of the nipple should be an un- 
interrupted cone in shape. 

All taps and fittings of a jet, especially of a mixing or high- 
pressure jet, should be of the best manufacture and finish. 
The nozzle often is removable from the mixing-chamber, a 
very good plan, but the collar by which the junction is made 
must fit accurately and be tightly screwed up. There should 
never be any leakage even under the highest pressures. 

Jets are sometimes made " interchangeable " ; that is, sup- 
posed to act either as blow-through or mixing by a change of 
nozzles. We do not recommend this kind of jet, the same 
kind of " chamber " is not well adapted for the two systems. 

As ether tanks, or " ether saturators," and saturators for use 
with other volatile substances, are frequently used, and as they 
are very convenient when hydrogen is not obtainable we shall 
describe one or two such contrivances, drawing attention, 
however, to the danger incurred if carelessness creeps in, and 
to the fact already mentioned, that we do not expect any gain 
of light over that of a good mixing jet. The principle on 
which ether saturators all work is this : Oxygen is passed over 
ether, which may be sulphuric or petroleum, and in passing is 
saturated with the vapor of the ether and then is conducted 
to the jETtap of the jet, while part of the oxygen that reaches 
the tank passes directly and unsaturated to the O. tap. In 
some tanks the ether is in the liquid state, lying in divided 
chambers, over which the oxygen passes ; in others, flannel or 
a similar substance is thoroughly wetted with the ether, and 
the oxygen passes over or through layers of the damp material. 
The Broughton ether tank, for instance, is a simple oblong 
vessel of copper, divided longitudinally into two parts, the 
lower part having a number of compartments wherein the 
ether lies. The oxygen passes over the ether into the upper 
part of the tank, and thence to the jet, while, by means of 
a T-piece at the entrance to the tank, part of the oxygen is 
conveyed direct to the jet, while the rest is charged with ether 
vapor, as described. The Ives saturator is different in form 



42 



THE OPTICAL LANTEEN. 



from the Broughton, and the Ives contains flannel impreg- 
nated with the volatile liquid, but the principle is the same. 




Figure 25 (a) shows Broughton's, Figure 25 (b) shows Ives' 
saturator. 

In both cases, O. represents the oxygen supply, H. the 
saturated oxygen tube leading to the H. tube of the jet. 

Quite recently Mr. A. W. Scott has introduced a saturator, 
wherein he uses benzole, benzoline or gasoline; the principle 




Fig. 25 (*). 

of saturating the oxygen remains the same, but the manner 
of carrying it out differs somewhat in Mr. Scott's contrivance 
from the saturators just described. 

Figure 25 (c) is a section of Mr. Scott's gasoline saturator. 
The central chamber (shaded in the figure) contains flannel, 
which is impregnated by pouring the volatile fluid in at E. 
F is a "night-light" or small candle, used to heat the air 
around the fluid slightly. C. is connected with the oxygen- 
supply, B. with the oxygen tap of the jet, A. with the hydro- 
gen tap. As before, part of the main oxygen supply goes 
direct to the jet, part passes through gasoline, benzoline, 
or benzole vapor and thence to the jet. 

"With all saturators, when the light is to be extinguished it 



THE OPTICAL LANTERN. 



43 



should be done at the jet taps first ; if the gas supply is cut 
off at the bag or cylinder first, a small explosion will probably 
occur, a " pop " at least. 




Fig. 25 (<■). 
A "safety-chamber" should be used with saturators always ; 
this chamber may be the " mixing chamber " of the jet or near 
it, and the best material for packing it appears to be pumice 
stone in small pieces. It need hardly be said that the greatest 
care must be taken not to overturn a tank containing liquid 
volatiles, nor must any oxygen or other gas be pressed into the 
liquid itself. 




CHAPTEE VIII. 

JETS CONTINUED, DETAILS AND ACCESSORIES, 

TUBES AND TAPS FOR GASES LIME 

TURNERS, JACKET. 

It is always important to know at the very instant, and 
without possibility of mistake, which tube conveys the oxygen 
and which the hydrogen. Often, too, it is important to know 
by touch as well as by sight. Accordingly we recommend 
that one tube be blackened, the other left bright. Further, 
we advise that the two taps be of different shapes. Our own 
practice is to have the II tube black and its tap the common 
T-shape, while the O tube is bright and has an " arm-tap," 
such as seen in Fig. 24, which, moreover, we roughen or nick 
with a file. 

As the impact of the burning gases on a small area of the 
lime sooner or later bores a small hole in the lime — in techni- 
cal language the lime becomes " pitted 1 ' — or a " pit " is formed 
— and as this pit not only spoils the illumination but endangers 
the condenser, it is important to have a good system and 
apparatus for turning constantly or frequently a new part of 
the lime's surface to the action of the gases. The more 
powerful the blow-pipe heat the more frequent the necessity 
for a fresh lime surface. With the most powerful jets it used 
to be considered almost essential that the lime be constantly 
turning ; this was usually effected by clockwork ; with an 
average mixing jet and pressure the lime ought to be turned at 
least once every minute. But with a blow-through jet of ordi- 
nary power the lime-point does not attain its full incandescence 
till the gases have played upon it for, say, one second, and in 
this case the lime ought not to be turned so often. We have 
some very crude methods of turning the lime, some very neat 



THE OPTICAL LANTERN. 45 

ones. The old inconvenient method consisted in opening the 
lantern door and turning the " chair " round with the hand. 
Nowadays the chair is usually operated by a rod and some 
kind of cog wheel arrangement, the rod being long enough 
to protrude backwards to the outside of the lantern, in a 
manner shown in all our figures of lanterns with jets. Mr. 
Place, of Birmingham, designed an exceedingly' good quick 
motion action for the lime turning. Messrs. Newton further 
improved this by adding a check-action, or click action, the 
effect of which is that the points presented consecutively 
to the gas-jet follow a regular spiral course round the lime- 
cylinder, the eighth pit being directly below the first ''see 
Figs. 22 and 25). 

Fig. 25 shows a detail not yet explained, but in the opinion 
of competent judges a valuable addition to a jet. Below the 
jet proper may be seen a rod with a T-piece at one end and 
operating toothed wheels under the jet tubes. In each of the 
two tubes is placed an extra tap, that on the oxygen tube being 
a complete " cut-off," while that on the H tube is a " bye- 
pass " allowing a small quantity of H to pass even when the 
tap is turned off as far as it will go. The two taps work 
equally, and are simultaneously operated by means of the rod 
and the toothed wheel attached to it. If now these two taps 
be turned full on by means of the cross, or T-piece, and if the 
light be then adjusted at its best by means of the ordinary jet 
taps, the light may be turned down by the " cut-off " arrange- 
ment, the hydrogen will continue to burn a small flame, and 
the lime will be kept hot, and when the full light is required 
all that is necessary is to turn up the " cut-off " arrangement 
to its full extent. Thus, 1st, gas is not wasted ; 2nd, an expe- 
rienced hand may regulate the light, turn it down and leave 
it ; any person, however inexperienced, may then turn up the 
"cut-off" and find the light just as the expert arranged it; 
3d, the proportions of the gases being maintained the total 
brilliance of the illumination may be regulated by one motion ; 
where miscellaneous slides are being passed through a lantern, 
some dense, others thin, this facility of, regulation will be 
found of value. This "cut-off," devised by the author, is 
made by Messrs. Newton, of London, and others. 



46 



THE OPTICAL LANTEKN. 



Messrs. Oakley, also of London, make an arrangement 
similar in intention but dissimilar in execution. 

Lime-cylinders are very apt to crack during an exhibition, 
and a lime once cracked never gives a perfect light. Consid- 
ering that draughts of cold air acting on the lime causes this 
cracking, undoubtedly the case often, Mr. E. G. "Wood, of 
London, devised a " shield " of metal a little wider than the 
lime-cylinder, and long enough to protect the cylinder even 
when turned to its highest point. Of course an opening in 
the side of the shield allows the gas to impinge upon the lime. 

Most persons of a mechanical turn of mind will notice the 
very insecure way in which the jet is usually fixed to the pin 
or standard forming part of the " lime " tray. The jet is 
fixed in such a position that the slightest force on the long 
arm of the lever must uncentre the whole jet. Few attempts 
have apparently been made to improve this matter, but Mr. 
Pumphrey, of Birmingham, Eng., has an arrangement worthy 
of notice. Fig. 26. 




Fig. 26. 

Here, in Mr. Pumphrey's " mechanical stage," we have the 
usual backward and forward sliding motion of the tray, and 
also a raising and lowering rack as well as a " traverse " and 
a double grip of the jet. The figure also shows the receptacle 
for lime-discs, seldom used now so far'as we know. 

The writer effects a double grip on the jet by means of a 
pair of "jaws" so arranged as to clamp the jet by the mixing 
chamber to the bottom of the lantern when once the centre 
and focus are found. 



CHAPTER IX. 
PREPARATION OF OXYGEN AND HYDROGEN. 

Chlorate of potash heated to a fairly high temperature 
will yield oxygen, but the evolution of the gas in such a case 
is apt to be unsteady and difficult to control. If, however, 
"black oxide of ^manganese " (manganese dioxide), is added 
to the potash salt, the gas comes off at a lower temperature 
and more steadily. But when the manganese is used a certain 
amount of chlorine comes off with the oxygen ; and this 
chlorine, being destructive to india-rubber bags, and even to 
metal fittings as taps and tubes, should be as far as possible 
eliminated before the oxygen reaches the bag or gas-holder 
where it is to be stored. We heat our chlorate of potash and 
oxide of manganese — the mixture being sometimes called 
"oxygen mixture" — in a retort made of metal; the oxygen 
with its accompanying chlorine is passed through water 
containing a chemical which abstracts the chlorine, thereafter 
the oxygen is sometimes dried over sulphuric acid, sometimes 
passed direct into the bag or other receptacle for storage. 

The chlorate and manganese do not combine in the oxygen 
mixture, nor does the manganese undergo any permanent 
change during the heating, but is probably converted for the 
time being into permangate of potash, which is at once decom- 
posed. After the operation of gas-making the manganese 
oxide remains, and can be used again if it is considered worth 
the trouble of washing and drying what remains in the retort. 
Manganese dioxide gives off oxygen at a red heat, but is not 
so far as we know used for this purpose. 

The usual instruction for making oxygen gas is to take four 
parts of KCIO3 (chlorate of potash) and one part of Mn0 2 
(manganese dioxide) and mix them. These proportions are 
a very good average, but our own practice is to take the 



48 THE OPTICAL LANTEKN. 

required amount of KC10 3 and stir up with it Mn0 2 until 
every crystal of the former is black with the latter. The 
mixing should be done with a wooden or bone rod or spatula, 
not with a metal instrument. As a rule the KC10 3 is pure 
enough as sold, but sometimes the ignorant or unscrupulous 
trader adulterates his Mn0 2 with soot or charcoal, and some- 
times by accident sulphide of antimony is substituted for or 
mixed with Mn0 2 . Either soot or the antimony being 
present in the retort with the chlorate would be a source of 
great danger, but the mixture may be very easily tested. If a 
few grains of the mixture be placed in a test tube and held 
over a flame, and if on gentle heating little sparks are seen 
accompanied perhaps by a slight crackling, the mixture is 
correct. But if there is a flash or a little explosion, the 
mixture is dangerous. When KC10 3 is bought in bulk there 
is almost always mixed up with it a quantity of organic 
matter, bits of wood, paper, we have even seen moss ; these 
must be carefully removed, as they might lead to trouble in 
the retort. 

One pound avoirdupois (sixteen ounces) of the oxygen 
mixture will yield four cubic feet of oxygen — theoretically it 
ought to yield rather more, but we never could get more out 
of it, using ordinary chlorate of potash of commerce, which 
is good enough for the purpose. Some years ago it was 
suggested that in place of or in default of Mn0 2 , common 
kitchen salt might be used. And certainly this is the case ; if 
the chlorate be pounded along with the salt, or the salt care- 
fully mixed with the pounded chlorate, the oxygen comes off 
very gently and steadily, and the yield of oxygen in proportion 
to the chlorate is good. Iron rust or sand, and doubtless 
other substances may be used for the same purpose as Mn0 2 , 
and we note that Mr. Hepworth* uses both Mn0 2 and salt. 
We find nothing works better than KC10 3 blackened with 
Mn0 2 . 

The mixture being ready is placed in the retort. Figure 
27 shows the kind of retort that we can recommend from 
experience. 

* "The Book of the Lantern." By T. C. Hepworth, F.C.S. Wyman : 
London^ 1889. 



THE OPTICAL LANTERN. 49 

The body is conical in section, made of wrought iron, 
brazed and riveted. The neck screws on to the body and a 
washer of mill-board or felt impregnated with asbestos is 
placed between body and neck. Sometimes the bottom is 
made of copper, this costs more and is unnecessary. At the 




Fig. 27. 

top may be seen a small cylindrical projection ; in this is 
placed pretty tightly a cork, and so a safety-valve is obtained. 
If any stricture occur in the gas-way the cork will be blown 
out. The neck of a retort should never have a square turn, 
but always be rounded, as in Fig. 27. 

No violent heat is needed to drive off the oxygen, in fact, 
too much heat, especially at first, retards operations, seeming 
to cause the mixture inside to form a cake, with an outside 
more or less impervious to heat. The retort may be put on a 
dull red fire, but we prefer a gas burner such as some made by 
Fletcher, of "Warrington, Eng., under such a name as " radial " 
burner. A coke stove makes a good heating appliance, 
and even an apparatus with a very large wick, known as a 
" lamp stove" may be employed. But the radial gas burner is 
probably the best. If by sinking the retort in a suitable 
receptacle over the burner the heat can be made to surround 



50 



THE OPTICAL LANTERN. 



the retort, so much the better, especially if there be a large 
quantity of " mixture " in proportion to the size of the retort. 
From the retort the gas passes along the neck-tube to a 
purifying bottle. This may be any wide-mouthed bottle, 
having a bung through which pass two tubes of metal. The 
retort neck is connected by a rubber tube with one metal tube 
of the purifier, and this metal tube dips below the surface of 
water in the bottle to near the bottom of the bottle. The second 
metal tube has one end inside the bottle but well above the liquid 
and the other end is outside the bottle, and connected by a 
rubber tube, either with a second bottle or directly with the 
gas-bag or tank. The first purifying bottle should be about 
two-thirds filled with water, to which a small quantity of 
caustic soda has been added, or caustic potash. This and the 
water remove nearly all the chlorine that comes over with 




Fig. 28. 

the oxygen. If a second bottle containing a similar solution 
be used the gas will, after passing through it, be practically 
free from chlorine, as may be verified by the absence of the 
peculiarly unpleasant smell of the latter gas. Carbonate of 
soda or of potash will answer almost as well as the caustic 
alkalis. Figure 2S shows an ordinary purifying bottle. "We 



THE OPTICAL LANTERN. 51 

prefer a good india-rubber bung to any metallic cap or fitting, 
as the rubber bung acts as another safety-valve. And we 
prefer a glass purifier to an opaque one, as it is well to observe 
at what rate the gas is coming off as shown by the bubbling 
inside the bottle. 

It is well sometimes to dry the oxygen before putting into 
the storage receptacle. For this purpose the second purifying 
bottle may have a small quantity of sulphuric acid put into it, 
but of course in this case both ingress and egress tubes are 
kept above the surface of the acid. This process of drying is 
rarely necessary, a second solution of alkali, or even a quantity 
of plain water is, however, of considerable utility. 

Practice of Oxygen Making. — Having mixed the KC10 3 and 
MnO g as described, and placed the mixture in the retort, and 
the retort on the heating apparatus, we connect our system all 
along, the retort with the first purifier, this purifier with the 
second if we are to use two. But we do not connect the last 
purifier with the gas bag or gas holder. Before connecting 
any two parts of the apparatus we must blow through each 
separate part to make sure that there is no obstruction in the 
gas-way. The neck of the retort is to be screwed pretty 
tightly on to the body, the safety-valve cork pressed firmly but 
not violently home, and the retort is to be supported in such a 
way that it cannot be overturned, one fatal accident is said to 
have occurred through the overthrow of a retort, and 
consequent clogging up of the neck by the heated mixture. 

As the retort is gradually heated a slight crackling may be 
heard, and probably an occasional bubble will rise in No. 1 
purifier. Presently a slight rush of air may be perceived at 
the free end of the arrangement, viz : at the end of the rubber 
tube which, later, is to be connected with the bag. 

We now light with a match a piece of brown paper or some 
such material, blow out the flame if any, and hold the smould- 
ering paper in the current of air passing from the free end of 
the tube. A lighted pipe or cigar is useful and often handy 
for this test. The presence of oxygen in the current will 
easily be verified by the paper or cigar bursting into a pecu- 
liarly bright flame. The gas-bag — if such is to be used — 



52 THE OPTICAL LANTERN. 

should previously have been as nearly as possible emptied of 
air ; to do this, fold the bag up with the tap open and kneel 
on it or otherwise press all the air out, shutting the tap 
instantly. The moment before connecting the tube with the 
oxygen now issuing from it, the tap of the gas-bag is to be 
opened ; .this is sometimes forgotten, and leads to a safety- 
valve being blown out. The bag should be on a level higher 
than the purifiers, for sometimes when gas comes off rather 
violently Water gets blown into the tube leading out of a puri- 
fier, and water in a bag would be rather out of place. The 
oxygen is now allowed to come off steadily ; if ever the evolu- 
tion becomes violent the heat should be lessened either by 
lowering the gas in the burner, or by taking the retort off the 
fire or stove. The heating must not be violent at first, as 
above stated. Yery likely after a time the evolution of oxygen 
will stop and appear to be finished, but if the proper quantity 
of oxygen has not been obtained (see above) the retort may be 
shaken or get a smart knock, or may be turned partly on one 
side; and often even if the retort be left alone the oxygen 
will of its own accord begin to come off again though the 
pause may have extended over several minutes. The bag may 
be filled " drum-tight," it will probably be less tight after the 
oxygen in it has cooled. A little deftness is needed if the bag 
be full before the oxygen has ceased to come over. When the 
bag is nearly full the heat on the retort should be lowered, but 
not entirely removed ; when it is desired to stop operations, 
the gas-bag tap is closed, the tube connected with it instantly 
removed ; the heat removed from the retort and the tube 
below water in the first purifier immediately pulled out of the 
water. *If this be not attended to, the water from the purifier 
will be sucked back into the retort, which will be burst in all 
probability. Retorts should not be of cast iron, but of wrought 
metal which will rip but not fly to pieces. 

As soon as the retort is cold it should be well washed out 
with water ; changes of water are to be put in until the water 
comes out quite clear. The manganese, as stated, may be kept 
by filtering, in any case it is apt to make a very nasty mess if 
the contents of the retort are poured out into a sink. The 



THE OPTICAL LANTERN. 



53 



"beginner should examine the contents of the retort at this 
stage ; if he finds any unaltered chlorate of potash he will 
know that his mixture was not exhausted. Large cakes in the 
retort are a sign of over-violent or over-rapid heating. 

Hydrogen Making. 

Hydrogen gas pure is stated to give with the lime a better 
light than ordinary carburetted hydrogen, such as is used for 
house illumination. Our own experience leaves us in doubt 
on this point, but our trials were made with a kind of house 
gas not common, as it was made directly from paraffine oil' and 
merely purified by water. 

It is very easy to make hydrogen practically pure. Scraps 
of zinc are placed in the bottom of a "Woolff bottle having two 
necks. Into one neck is fitted a rubber bung bored to take a 
long thistle-head funnel reaching well down into the bottle. 




Fig. 29. 

The other neck has also a bung fitted with a bent tube, reach- 
ing an inch or two inside the bottle and connected outside with 
a water purifier as for oxygen, which purifier is connected with 
the hydrogen storage receptacle. Into the thistle-head funnel 
dilute sulphuric acid is poured (acid one part, water four parts), 
when the hydrogen will be given off at the delivery tube. 
The funnel-point must, of course, be below the surface of the 
acid liquid in the bottle. The purifier contains plain water. 

A flask with double-bored rubber bung may be used in place 
of the Woolff bottle, as in Fig. 29. 



CHAPTER X. 

STORAGE OF THE GASES— I. IN BAGS. PRESSURE- 
BOARDS, WEIGHTING, ETC 

We have under this heading practically only two systems to 
consider, viz. : Storage in "gas-bags" or tanks, and storage in 
metal cylinders at high pressure. 

"With gas-bags "storage" is hardly a proper word to use, 
because the less time the gas is stored in a bag the better for 
the bag and for the light. But we may have to put the gas 
into a bag for use immediately or soon. For a person making 
his own gas at home there is the alternative of a bag or a 
metal "holder" or "tank," and in practice the bag will be 
found the more convenient if the " lantern-display " is to take 
place away from home ; but if the gas is to be used at home, 
then the metal holder has the advantage. 

Gas bags are made of " india-rubber cloth," that is, of rubber 
lined outside and inside with some kind of cloth. Generally 
the inner lining is of canvas, the outer of a stuff known as 
"twill." A gas bag is usually of wedge-shape, and in the 
middle of the thin end of the wedge is let in a tap. The 
manner in which this tap is let into the stuff of the bag often 
makes all the difference between a good bag and a bad one. 
The tap should screw into, or be soldered into, a large, pretty 
thick plate inside the bag, if the plate is not thick but inclined 
to have sharp edges, the bag will sooner or later be cut by it. 
Two qualities of bags are usually sold, the better quality is 
invariably the cheaper in the end. But the better quality is 
not always dearer to purchase, for we know second-rate bags 
made expensive by the quality of the outer cover. The best 
bags the writer has ever used are plain black outside, and have 



THE OPTICAL LANTERN. 55 

taps properly let in, and so made as to be locked with a pad- 
lock when it is desired. Figure 30 shows one of these taps. 



Fig. 30. 



Bags are made in various sizes, a convenient bag holds 
seven cubic feet of gas, and may be thirty-six inches long, 
twenty-eight wide, and have a wedge-depth of twenty-four 



Fig. 31. 

inches. For a very long lecture or for unusual pressure the 
bag may hold about ten cubic feet, and be 40x32x(wedge)28 
inches. Gas bags used to be made flat on top, and are some- 
times so made still, but the wedge is the usual and the better 
shape. 

In order that the pressure may be applied to the bag or bags 
we place them between " pressure-boards." If we are using 



56 THE OPTICAL LANTERN. 

the blow-through jet we put the oxygen bag under pressure 
and use hydrogen from the main, so also with the oxycalcium 
light, one bag only is required. With ether-oxygen and ben- 
zine-oxygen one gas only, oxygen, is used under pressure. In 
these cases a single pair of pressure-boards is required. But 
as double pressure-boards answer also for single bags, we 
recommend the purchase of double boards, a figure of an 
example of such is here given. Fig. 31. 

The bag or bags are placed in the jaws of this contrivance, 
the taps projecting through the hole cut out for them at the 
apex of the board-wedge. The bags are to be pushed well 
home into the jaws of the pressure-boards, and the partition, 
usually of strong sail-cloth, separates the two bags if two are 
used, or is allowed to lie flat if only one bag is in use. After 
the bags are in position the strap at back is to be tightly 
pulled and securely buckled, to prevent the bags from 
springing backwards. The support seen in front of the board 
will be found necessary when the bags are large and full; 
but the support is hinged and folds out of the way when not 
required. Pressure-boards must be of strong wood, and the 
hinges at the apex must be very strong. On the ledge seen 
near the top of Fig. 31 we place the weights. These weights 
may be twenty -eight pounds or fifty-six pounds each, and 
should be flat and not round. If any danger can be said to 
exist in working the mixed gas jet it may be said to lie in the 
possibility of the weights falling off the pressure-boards and 
so causing a suck-back of gas. Therefore the weights should 
be tied on to the ledge in some secure way, more especially if 
they are round weights. We recommend the worker at home 
to have fifty-six-pound weights made of lead or iron with a 
very broad base. < 

In computing or quoting the pressure on the gas we have 
nothing to do with the size of bags used, though it is very 
common to see quotations on this basis of calculation. What 
we have to measure is the area of the top of the pressure- 
boards. This in square inches divided by the number of 
pounds weight bearing on the top gives the pounds of pressure 
that concerns us. A top 40x28 inches will suit the seven feet 



THE OTPICAL LANTERN. 57 

bag suggested on page 55, and if we place on our boards one 
hundred-weight we have a pressure of ten pounds per square 
inch, which ought with separate gases to give a very fine 
light indeed. We figure a pair of boards and bags in position. 




Fig. 32. 

For a blow-through jet, fifty-six pounds weight on the 
pressure boards (40x28 inches) ought to suffice. Gas must not 
be kept long in rubber bags, such prolonged storage is bad for 
the bags and bad for the light. The sooner the gas is used after 
it is made the better. Oxygen kept twenty-four hours in the 
best bag will give a very poor light, as we have repeatedly 
found. Even four hours has in our experience made a marked 
difference on the light. Presumably hydrogen also deteriorates 
in the same way, probably to a greater extent. Even such a 
septum as india-rubber can not prevent diffusion by osmosis, 
and, moreover, the bags themselves suffer seriously by having 
gas kept in them. Immediately after the " display " or lecture 
is over the gases ought to be forced out of the bags, the bags 
being folded and kneeled upon after removal of the taps. If 
an old bag's inside be examined the effects of oxygen will 
easily be observed. 

Oxygen may be stored for a considerable time over water in 
a metal tank, but even under these conditions it deteriorates 
after a time. Moreover, if the metal of the tank be not 
thickly tarred or painted the oxygen will attack it. We need 
not figure a metal tank which consists merely of a small edition 
of a gas-holder, or what is for some reason unknown to us 
called a " gasometer," so often seen at public gas works. 



58 



THE OPTICAL LANTERN. 



Before leaving the subject of bags we would say that if in 
cold weather they become very stiff they should be warmed 
before filled with gas. The taps should be kept working 
" sweetly " with vaseline, or some such lubricant. If a bag 
happens to get a hole in it, the owner would do well to have it 
repaired by an expert, rather than at home. 




CHAPTER XI. 

STORAGE OF GAS (2). CYLINDERS. REGULATORS. 

GAUGES. 

Since the manufacture of oxygen, and its storage in metal 
cylinders at high pressure became in Britain an established 
and successful industry, the use of gas-bags has to a great 
extent fallen into desuetude, and, indeed, when we compare the 
advantages and conveniences of the two systems of storage 
there is not much wonder that bags are gradually dying out. 
It is long since first oxygen and other gases were stored in 
cylinders, but when the Brin's Oxygen Company started to 
make and trade in oxygen at prices far below the previous 
quotations, the use of their gas and metal cylinders rapidly 
gained the ascendancy, and "compressed" gases will probably 
oust gas bags entirely in a few years. Another matter that 
greatly popularized the cylinder system was the invention by 
Mr. Beard of a regulator so effective, simple and cheap, that 
we do not hesitate to stamp it as one of the most satisfactory 
inventions ever made in connection with the optical lantern. 

Barium monoxide gently heated in air takes oxygen from 
the air and becomes barium dioxide, Ba O becomes Ba 3 . 
If now the air supply be cut off and the temperature of 
the Ba 2 raised, the Ba 3 gives up part of its oxygen 
and reverts to its original state Ba O, and these changes can of 
course be repeated time after time by regulation of air and 
temperature. All this has long been known, but nobody seems 
to have • been able to devise machinery for utilizing these 
reactions until Brin's Oxygen Company came before the 
public with their patent machinery. On Mr. C. H. Bothamley's 
authority the writer states that the following are the requisites 
for success in this system of oxygen-isolation. (1) Proper 
regulation of temperature. (2) Perfect purification of the 



60 THE OPTICAL LANTEEN. 

air from C0 2 carbon dioxide. (3) Presence of a certain 
definite amount of moisture in the air. (4) Maintainance of a 
low pressure in the retorts. The furnace is ingeniousl j made to 
regulate its own temperature by the expansion or contraction 
of metal bars acting on "dampers." C0 2 is eliminated from 
the air by caustic soda and lime ; the air is then dried or 
damped according to necessity, and passes over the hot Ba O in 
long convoluted cylinders, the oxygen is got from the now 
Ba0 2 by raising the heat, the nitrogen having been collected 
or allowed to escape. The oxygen is finally pumped into 
metal cylinders. It appears that the awkward part of the 
business is to get rid of the nitrogen. Oxygen made by this 
process often is accompanied by nitrogen which does not 
improve the oxygen for our purpose. 

Mr. Orchard, of Kensington, London, prepares and com- 
presses gas which, on analysis, has shown 98 per cent, of pure 
oxygen as tested at the Royal Institution. Probably it would be 
impossible to produce a purer article than this and impossible 
to sell it at so low a price as the barium-produced oxygen, but 
the light produced by the purer gas is decidedly superior 
in quality to the other. Brin's gas, however, is quite good 
enough for all ordinary purposes, and its cheapness, together 
with the facility with which we can obtain it in this country 
(Britain) gives it a strong hold on our attention. The writer 
uses Brin's gas almost invariably.* 

The oxygen and also hydrogen are forced at very high 
pressure into metal cylinders. These cylinders are of wrought 
iron or steel, and should be tested to a point of pressure 
greater (say three times) than what is to be actually used. 
In England the cylinders are usually charged to a pressure of 
" 120 atmospheres," or 1800 pounds to the square inch. To 
give an idea of the result we may state that in a cylinder thirty 
inches by five and three-eighths, weighing twenty-eight pounds, 
we can have forty cubic feet of oxygen ; while ten feet of gas, 
ample for the most prolonged lecture, can with ease be carried 
in one hand. 

* Lately the Brin Company has improved its machinery, and now 
obtains oxygen with little or no nitrogen. 



THE OPTICAL LANTERN. 



61 



It will easily be understood that when we have gas under 
such pressure as 1800 pounds on the square inch, there is a 
certain amount of difficulty in managing it, tubes are apt to 
be blown off, and there is a danger of leakage at all points not 
solid metal. The valves of these cylinders, therefore, require 
to be very strongly and accurately made, and a system for 
regulating the pressure of gas outside the cylinder becomes 
necessary. The latter desideratum is thoroughly well fulfilled 
by a small and simple apparatus, Beard's patent gas regulator. 
By using his contrivance, which is fixed on the valve tube of 
the cylinder, we can even turn on the gas full without opening 
our jet taps and without tying our rubber tubes to our jet 
tubes. This of itself is an enormous gain, for we can turn on 




Fig. 32 (a). 

the gas at the cylinder and then proceed to turn up the gases 
in proportions as required at the jet. Provided there is suffi- 
cient pressure of gas in the cylinder to keep the regulator 
open, the gas comes in a steady stream. As made at present 
the regulator is set for one maximum of pressure, but we have 
suggested to Mr. Beard a method by which he may make 
each regulator adjustable within fairly wide limits to differ- 
ent maxima of pressure. Figure 32 (a) shows the nature of 
Beard's regulators. Our frontispiece shows two gas cylinders 



62 THE OPTICAL LANTERN. 

with Beard regulators and also a pressure gauge. This gauge 
is frequently useful to show the rate at which the gas is being 
used, or to show at any time what amount of pressure, 
and consequently — the internal dimensions of the cylinder 
being known — what amount of gas there is in the cylinder. 
A cylinder fitted with these two contrivances may be put 
down as perfectly suitable and convenient for our purposes. 
"We give here a table for use along with a gauge ; the figures 
are for Brin's cylinders and may be taken as sufficiently accu- 
rate for any cylinders of the same type as used in Britain. 

It must be noted that in adding regulators and gauges to 
our gas-storage system we are always increasing the chances 
of leakage ; every joint must be very tightly screwed up by a 
" spanner" or other instrument of like nature. 

A rule usually observed, and most useful, is that the two 
cylinders for O and for H are painted in different colors. 
The oxygen cylinder is generally black or very dark, the 
hydrogen a bright red. 

On next page we give a table showing capacity, size, and 
pound-pressure of Brin's cylinders. 



THE OPTICAL LANTEBN. 



63 



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Capacity of 

cylinder 

in cubic feet. 



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CUBIC FEET OF GAS. 



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CHAPTER XII. 
SCREENS AND FRAMES. 

We have now to deal with a part of our system which the 
practical worker will find much more important than the inex- 
perienced or mere theorist might imagine. A good " screen " 
or surface for receiving the image makes a great difference in 
the results, a bad screen may easily absorb or disperse uselessly 
25 per cent, of the illumination. 

A screen ought to be opaque, white, and matt, not translucent 
(except for certain almost obsolete arrangements), nor " shiny," 
nor yellow. At one time it was not uncommon, and even yet 
it may at times be necessary to use a translucent screen, in 
this case a very thin linen screen is used and is almost of 
necessity kept damp. But such a system is comparatively 
poor, and ought to be resorted to only when the other is not 
available. 

If the same room is always to be used for the lantern work, 
portability of the screen is of no moment, and in such a case 
we can easily attain to a perfect receiving surface. A smooth, 
plastered wall is as good a surface as can be got. A stout 
fabric, such as canvas, may be faced with paper, or it may be 
heavily coated with sized white pigment. In mixing the pig- 
ment a little blue should be added to kill the yellowness of the 
whitening. If the screen is required to roll up like a school 
map, some care is required in mixing the pigment to make it 
in such a way that there shall be no cracking when the screen 
is rolled up. This is a matter for experiment ; if when the 
screen is dry after painting the white can be rubbed easily off 
by the finger, more glue is required ; if the surface is at all 
glossy, there is probably too much glue. For painting, the 
canvas is stretched on a frame, laid face upwards, and the pig- 
ment laid on as evenly and as rapidly as possible. 



THE OPTICAL LANTERN. 65 

Mr. Hepworth's directions are somewhat as follows : Tack 
good unbleached calico to a frame so that the seams, if any, 
lie horizontally. Secure first the four corners with tacks to 
the frame, then nailing one side at a time to the frame. Give 
the sheet a good coat of the best size, melted by heat with its 
own weight of water. The sized sheet is allowed to dry, and 
thereafter is painted with whiting and melted size. The 
whiting, water, and size mixture should when first made have 
the " consistence of cream," when cool it will be like a thin 
jelly. Mr. Ilepworth then places the frame upright, and pro- 
ceeds to work the brush charged with the paint up and down 
and sideways, so as to avoid leaving any lines upon the surface. 

We are informed that the best of all "pigments" to mix 
with the size is zinc white ; we have not ourselves tried this, 
but believe that the claim for it is well founded. 

Facing a screen with paper requires, we fancy, an experi- 
enced hand, the paper cannot be got in large enough sheets, 
and the laying down of the edges where the sheets join is no 
easy matter. Of course, if we only require a small screen, as 
for showing small images to small audiences, single sheets of 
paper may be procured of sufficient size, and only require at 
the most to be attached to suitable supports. 

Lp to (say) 10 feet square the advantages of a "faced" or 
sized screen are so great as to quite balance the awkwardness 
of a 10 feet roller for transportation from place to place. Up 
to 10 feet we therefore recommend such a screen, even for 
traveling. But beyond 10 feet the length of roller required 
becomes very inconvenient, and we have to look for a screen 
material that may be folded up, and a method of so stretching 
it when in use as to obviate the creases naturally following the 
folding. To meet these desiderata, we use rather thick linen 
or cotton screens, and we stretch them on "screen-frames." 
Cotton seems to be less used than linen, so we shall confine 
our remarks to the latter. We believe that pieces of suitable 
linen can be got up to 10 feet square, bat, as we have said, we 
should prefer a faced or sized screen of that size. If we have 
to join pieces of linen for a screen, say, of 18 feet, we must 
avoid having our seam or joint near the centre. In such a 



66 THE OPTICAL LANTEKN. 

case it would be better to have a complete square of 10 feet in 
the centre, with four feet tacked on all round, rather than 
simply to make an irregular patch-work. As these screens are 
sold ready made in any usual size, we need not do more than 
point out that the seams should be as far from the centre as 
possible, and that they should run perpendicularly rather than 
horizontally on the picture. If a seam falls coincident with 
the horizon of a picture the effect is apt to be very unpleasant. 

There is no great variety in the designs of screen-frames, or 
"elevators," as they are sometimes called. A screen frame 
should be as light as possible consistent with strength sufficient 
to keep a screen quite taut ; it should take down into as many 
short pieces as is consistent with strength as above ; it should 
be capable of being rapidly put together and taken down ; it 
should afford facility for elevating the screen ; it should stand 
by itself, and it should be capable of being tilted slightly 
without chance of falling down. When the frame is tilted 
is the time when strength is required, for a tilted screen will 
" sag " forward under such condition unless the frame be strong 
enough to counteract the tendency to sag. We see it stated in 
print that the advantage of being able to tilt the screen is more 
theoretical than practical, but we must dissent from this 
statement, a tilt is frequently a conditio sine qua non of real 
success ; in our own experience the screen is tilted nine times 
out of ten. 

We figure an " elevator " such as we find perfect for our 
purposes up to a twenty-foot screen. The poles are two inches 
in diameter, the lengths about five feet each, the junctions are 
strong brass ferrules, the corners are solid metal. The lengths 
are so made that we need not use all of them at a time unless 
we wish, so that we may choose our size of screen up to twenty 
feet, increasing or diminishing by about four feet at a time, or 
two feet if the lengths are made on purpose for such a choice 
of sizes, (fig. 33.) 

At the foot will be seen strong cross-pieces, which form a 
base, and with the addition of the four straps attached to the 
lower part of the frame help to prevent the screen and frame 
from toppling over if tilted. The tilt is arranged and 



THE OPTICAL LANTEBN. 



67 



maintained by shortening and lengthening the front and rear 
straps respectively, and the frame works freely in the holes 
made for the purpose in the base cross-pieces, so that any tilt 
can be obtained in an instant. 




Fig. 33. 

There are various methods of mounting a screen on its 
frame in a hall preparatory to a lecture. Perhaps the most 
convenient way is to put the poles forming the top of the frame 
together, joining to these by the solid metal corners the first 
lengths of poles below the top bar, thus : 




to tie the screen by its tapes to these lengths, and then to join 
on the next lower poles, tying the screen to these in their turn 
till the whole is erected. By means of the bottom tapes, and 
those within reach with or without a ladder, the whole fabric 
is finally made taut. With a large screen we always fix guys 
to the metal corners at the top, and fix our guys to anything 
that comes handy, as a rafter or staples driven in by ourselves 



68 THE OPTICAL LANTERN. 

if the authorities permit. "We have a twenty-foot frame of 
four inch poles which always require guying, and»in any case 
a couple of guys when the sheet is tilted ease the strain on the 
pole ferules greatly. A very good adjunct to a screen is a pair 
of tasteful curtains, which may be simply hung on the top of 
the frame and drawn aside by simple means as the lecture is 
about to begin. 

If an opaque, or " faced " screen is to be used, nothing is 
required but two side arrangements to support the ends of the 
roller on which the screen is wound. 




CHAPTER XIII. 

SUPPORTS. SLIDE CARRIERS. READING LAMPS. 

SIGNALS. 



We naturally require some kind of stand upon which to 
place our lantern, and the larger and heavier our lantern the 
stronger must be our support. Sometimes stands are made 




Fig. 34. 



with sliding legs, so that the lantern may be reared up very 
high, but we see no use for any such height, in fact our highest 



70 



THE OPTICAL LANTERN. 



lantern, if we are using a double or triple, should be as little 
above the level of our eyes as possible. Even if we are with 
our lantern in the middle of our audience the lowest lantern 
needs only to clear the head of the sitter immediately in front, 
and if tilting the screen is inconvenient, mounting a ladder to 
see into our lantern is ten times worse. A business-like stand 
is shown in Fig. 34. 

But the kind of stand used in a photographic studio for 
carte and cabinet cameras answers very well. Such a stand, 
however, can hardly be called very portable, and if the box 
which carries the lantern when traveling be placed on an ordi- 
nary table, more especially if a canting table be added (Fig. 35) 
there will be no necessity for further impedimenta. 




Fig. 35. 

" Carriers " may be described as guides by means of which 
slides are passed through the part of the lantern in front of the 
condenser. It is important that slides should pass smoothly 
into and out of their position in the light-way, each slide must 
find the central position at once without any fumbling on the 
part of the operator, and when more than one lantern is being 
used, as for dissolving effects, it is essential to success that the 
slides should " register," i. e., should have coincident discs on the 




Fig. 36. 



screen. For multiple lanterns a certain amount of intricacy in 
the carriers may be necessary, but for a single lantern the simpler 



THE OPTICAL LANTERN. 



71 



the carrier the better. After using nearly every carrier known in 
the market the writer goes back to the simplest of all, the old 
" Chadwick," or failing that, one very nearly as simple, made 
by Mr. Place, of Birmingham. We figure both of these. 




Fig. 37. 



Both these carriers are so made as to take and to centre at 
once any ordinary size of slide. The " Chadwick" (Fig. 36) 
should be made of thoroughly seasoned, well smoothed wood. 
The " steps " are made to suit the three different lengths of 
slides in vogue at one time, one slide pushes the one in front 
through, one or other "step" being used as a guide for 
the hand that pushes the slide. In Mr. Place's carrier the slide 
is let in from above, a push on the metal projection seen turned 
up in the figure at once centres the slide, and on further push- 
ing the frame carrying the two slides slips along, and so, one 
slide is put away as another comes on. For dissolving lanterns 
" Beard's self-centering " carrier is as good as any. 

Some people dislike an interval of darkness, however short, 
between the pictures, some dislike a period of brightness on 
the screen. Many efforts have been made, and appliances 
contrived, to simulate dissolvino; effects with a single lantern. 
So far as we have seen, these contrivances are of no value ; if 
any such attempt must be made, the operator may hold in 
front of the lens while the slides are being changed a very 
finely ground piece of glass. For ourselves, we see no objec- 
tion, but rather an advantage, in the short space of semi-dark- 
ness produced by the use of a Chadwick or Place carrier. A 
blaze of light on a white screen for a short time is, in our 
opinion, most objectionable ; it tries the eyes and spoils the 
first effect of the pictures following. 



72 



THE OPTICAL LANTERN. 



There are many kinds of reading lamps, intended for nse 
by the person on the platform. Of course, the object is to 
throw light on the book or paper without illuminating the 
screen. By highly ingenious contrivances of this kind we 
have never been able to read comfortably, and we have repeat- 
edly been left in darkness during the performance. Since 
first we used a candle shade shown in Fig. 38 we have never 
used any other reading-lamp. 




Fig. 38. 



A signal between lecturer and lanternist is generally requi- 
site ; an electric communication acting on a muffled bell is the 
neatest device, failing that, we know nothing better than a 
gentle tap with a knife on a glass of water, or the click of a 
castanet d la Muybridge. 

All india-rubber tubing used for lantern purposes should be 
of the very best quality, and of a bore as large as may be con- 
sistent with fitting the taps and jet-tubes. The larger the bore 
the less the friction of the gases in the tubes, and diminution 
of friction helps greatly towards brilliance and steadiness of 
light. Tubes should not be unnecessarily long ; first, because 
redundant tubing means extra friction, secondly, because spare 
coils of tubing are inconvenient and apt to be trodden on and 



THE OPTICAL LANTERN. 



73 



to get in the way. Lastly, rubber tubing can hardly be too thick 
in the walls ; very thick-walled tubing is dear to buy but 
cheap to use. Rubber tubes should be pushed far up on the 
metal tubes to which they are u sprung," and it is always a 
good precaution to tie on the rubber tubes to the metal ones. 
Yarious kinds of boxes are made to hold lantern slides ; we 
like a box for each set of slides — each lecture, for instance; 
and the box should have no grooves, but may have a spring 
arrangement to push the slides up against each other in the box. 




CHAPTEK XIV. 

LANTERNS, AND ARRANGEMENTS FOR 
EXPERIMENTS. 

There are many scientific experiments for full comprehension 
of which we depend chiefly upon our eyesight, and it is easy 
to understand that if we can utilize the lantern for showing on 
a large scale the image of an experiment being made on a small 
scale, we have found a further use for our lantern. As a 
matter of fact many chemical, magnetic, optical and other 
experiments can very easily be shown to a large audience, 
though conducted on a small scale, and we may add to this the 
fact that it is possible to add to an ordinary optical lantern a 
projection microscope, polarising and spectroscopic apparatus, 
and various other optical appliances. 




Fig. 39. 

It is long since first attempts were made to project images of 
microscopic objects by means of a microscopical objective and 
suitable condensing lenses. The animal and vegetable contents 
of a drop of water have many times been projected with high 
magnification on a screen, fleas and other insects have many a 
time figured on ten feet discs, but lately considerable advance 
has been made in this line, and images of very minute 



THE OPTICAL LANTERN. 



75 



structures may now be projected by even immersion objectives. 
The more elementary form of lantern microscope is shown in 
our figure 39, and little explanation is required. 

The entire arrangement seen in our cut takes the place of 
the lantern projection lens. Behind the micro, slide is a 
rotating diaphragm with various apertures, and in practice the 
smallest aperture that will pass the entire image of the object 
should be used. This arrangement works very well with one 
low power, that power (or object glass) being the one for 
which the apparatus is intended, but as soon as one wishes 
to use a higher power the performance of the apparatus 
becomes faulty. A superior article is the outcome of Mr. 
Lewis Wright's experiment and ingenuity. Here (fig. 40) we 
have power of using and adjusting a microscope condenser, we 
have a "fine adjustment" for the micro, objective, as well as 
an alum cell to prevent the slide being damaged by heat. 




Fig. 40. 



A special condenser of the triple type is recommended for 
this projection arrangement. 

In like manner a polarising apparatus may be attached to 
the front of the lantern, the apparatus, figure 41, we have 
seen used with great success. 



76 THE OPTICAL LANTERN. 

Spectroscopic experiments may be demonstrated by means 
of an ordinary lantern with a slit-diaphragm attached to the 
front, prisms and collimator being placed on suitable stands 
between lantern and screen. With an electric lamp and a 
primary battery, and an apparatus designed by Mr. John 
Browning, the writer has carried out many interesting 
experiments on the spectra of metallic and other substances. 
Demonstrations on such lines take a firm grasp of the attention 
and interest of any audience. 




Fig. 41. 

For chemical experiments, and, indeed, for many others, an 
"open stage" is almost a necessity. Fig. 42 shows a lantern 
well adapted for such work. 

A so-called tank or cell, as seen in Fig. 43, may be placed 
in the open stage, and in this tank chemical reactions may be 
made to take place, the action being finely displayed on the 
screen. The same may be said of magnetic, electric, and very 
many optical experiments. If a sheet of orange or yellow 
glass be placed between the condenser and the tank a photo- 
graphic image may easily be developed, coram jpopulo, care 
being taken to use a very "slow" plate such as a chloride 
plate used for lantern slides. For these and other experi- 
ments with liquid in the cell a pipette should be used to drop 
in the reagents that determine the action, the cell having been 
previously nearly filled with one of the liquids. A very 
interesting series of experiments capable of being well shown 
by the lantern will be found in the contributions of the late 
W. B. Woodbury to the English Mechanic some years ago, 



THE OPTICAL LANTERN. 



77 



now edited by Mr. George Smith, of the Sciopticon Company, 
England. 




Fig. 42. 



"We figure on following page (Fig. 44) a good lantern made 
scientific for purposes by Messrs Newton, and intended to be 
used with an electric lamp. 

Figure 44# shows a double or biunial lantern, just brought 
out by Messrs. Newton, of London, and called the " Scientist's 
Biunial." The writer uses one of these with great satisfac- 
tion, both as a small and simple dissolving apparatus and for 
optical and other experiments requiring an open stage. The 
chief novelty is that the upper lantern can be turned up so 



78 



THE OPTICAL LANTERN. 



that the image is projected towards the ceiling, a plane mirror 
at suitable angle (see cut) is then fixed to the front of the 
projection lens so that the image is reflected on to the screen. 




Fig. 43. 



If, with the lantern in this position, we make a sketch on a 

piece of ground or clear glass in the stage of the upper lantern, 
our sketch will appear, as we draw it, on the screen. Many 




Fig. 44. 



pieces of apparatus, specially designed to illustrate certain 
optical experiments, are made to go with this lantern. The 



-■. 



THE OPTICAL LANTERN. 



79 



lanterns can be separated very easily, and each lantern is a 
very good and compact instrument in itself. 




Fig. 44a. 

The front carrying the lens can be entirely removed ; so 
also can the support with springs intended to carry the slide- 
carrier, when one is used. The condenser is 4^ inches diam- 
eter, and a good parallel beam can be projected by suitable 
manipulation of the lime jet. Altogether, this is a singularly 
ingenious and complete lantern. 



CHAPTER XY. 

PRACTICAL WORKING OF THE LANTERN, OIL 
• LAMPS, BLOW-THROUGH JET. 

Little requires to be said with regard to lighting and using 
an oil lamp in the lantern. The lamp itself must be thor- 
oughly clean outside and in, the wick or wicks accurately 
trimmed, and the full flame turned up not all at once but 
gradually ; the wick ought at no time to be turned so high as 
to cause a flare ; if once this mistake is made the lamp never 
seems to burn so well till all has been cooled down and the 
wick retrimmed. With paraffin lamps it is a common and 
commendable practice to put a lump of camphor weighing 
(say) one-half ounce with half a pint of the oil. This addition 
certainly whitens the flame. 

As a rule there is no adjustment for raising and lowering 
an oil lamp in a lantern, but we require by pushing the lamp 
forward or pulling it backward to get the flame as nearly in 
the focal point of the condenser as the size of our radiant 
will permit. This we judge by watching the disc of light 
thrown on the screen, colored edges on the screen mean that 
the light is not in focus. "With many-wicked lamps we are 
apt to have an ill-defined image of flame down the centre of 
the disc : we are informed that all lamps do not necessarily 
show this defect, and we can say that with all the lamps we 
have used of moderate pretension to good quality, the defect 
has been so slight as not to entail absolute failure. 

Our chief business, however, is with the lime light, and to 
it we now address ourselves. 

The "limes" are either turned from solid pieces of "hard" 
or " soft " limestone, or they are composed of a mixture of 
substances the nature and proportions of which are kept secret. 



THE OPTICAL LANTERN. 81 

The best limes so far as light-giving qualities are concerned 
are those turned from- the natural limestone, but for some 
reason they are never turned true, the hole up the centre is 
seldom true and the cylinder-shape is never accurately kept. 
The limes known as "Excelsior" are turned accurately 
enough, but are slightly inferior as radiants. As, however, 
the distance from nipple to lime is a factor of considerable 
importance in producing the best light, and as of necessity 
this distance varies as we turn the badly-shaped limes, the 
Excelsior limes may in the event give the better light. If we 
could get the " Nottingham " limes truly turned we should 
never use any other ; as matters stand we use Excelsior or 
Nottingham as they come to our hand, except for a very 
powerful light when the Excelsiors are not available. Prob- 
ably the best lime for the blow-through jet is the " Excelsior," 
which is too soft for the mixing jet. 

The limes are sold in hermetically sealed tin boxes with a 
quantity of powdered quicklime, or in glass bottles with luted 
corks and usually without the quicklime. The limes have a 
great avidity for moisture and will quickly go wrong if any 
moisture reaches them ; so we must take every precaution to 
prevent the access of damp. Various plans have been tried 
to secure the limes from damp, our favorite system is to wrap 
a piece of paper round each lime and dip it into "paraffin 
wax" just above melting point. Some makers send out 
their limes already rolled in paper, this gave us the idea of 
the paper, for without it the paraffin is apt to stick to the 
lime cylinder. Mr. Hepworth points out this, and suggests 
the precaution of keeping the paraffin wax at a low 
temperature. Opticians sell long brass tubes to hold 
one dozen, six, or three limes end to end, these 
tubes have well-fitting screw lids and are very 
useful. To one of these (Fig. 45) we added a little 
chamber in the lid, fitted with, fine gauze ; into this 

chamber is put calcic chloride, which is not without 
Fig 45 

use as a protection against damp for the limes. In 

any case a tin of limes once opened is no longer a safe recep- 
tacle for limes, more than once we have known a tin burst and 



82 THE OPTICAL LANTERN. 

ten or eleven limes lost under such conditions. At one time 
we used a wide-mouthed strong glass bottle, into which we put 
lime-cylinders quicklime and all as soon as we opened a tin, 
luting a tightly-fitting cork with melted paraffin; this was 
fairly satisfactory, but inferior to the brass cylinders mentioned. 

Before "lighting up" for a display two or three limes 
should be baked in an oven or on a hob ; one is placed on the 
lime-pin and the hydrogen at once lighted. The best distance 
from nipple to lime is a matter to be decided by experiment, 
a mixing jet usually has its nipple much closer to the lime 
than a blow-through, the distance for a mixing jet may vary 
from one-sixteenth to one-quarter of an inch, we have seen 
the approach so close as almost to amount to contact, and that 
in the hands of an excellent operator. In all jets there is an 
arrangement for adjusting the position of the lime with 
regard to the nipple ; of course the lime is moved, not the 
nipple. 

We shall now suppose, first, that a blow-through jet is to be 
used ; we have a bag full of oxygen between pressure-boards, 
or a cylinder full of the gas, while our hydrogen tube is 
connected with the gas supply of the hall or room. If we 
use a bag we must have a weight of say fifty-six pounds, to 
which we may add one of twenty-eight pounds in case of 
need. Before lighting anything we open our bag tap, and 
our O jet-tap connected with the bag by a rubber tube, 
having previously put our weight on the boards, and we 
allow the O to escape for about fifteen seconds to expel air 
from the gas way. If we omit this, our lighting up may 
start with a little "crack," which is unpleasant. Then we 
shut off the oxygen and light the hydrogen, keeping the 
flame about an inch and a half in height or less. Next we 
gently and gradually turn on the oxygen, till we get a fairly 
good light without much redness or scattering of the gases. 
We may at this juncture turn our lime round a few times to 
see that it keeps its distance from the nipple and also to let it 
get pretty well heated all over. Next we turn up the H a 
little, at once the flame will become red, a little more O will 
once more produce the blue and bright light, and so on we 



THE OPTICAL LANTERN. 83 

may go, H and O time about till at last we get to a point of 
brilliance that we cannot pass, and that may be taken as the 
best light our apparatus will give. Extra weight on the bag 
may or may not improve the light. 

If we are using O in a cylinder with a regulator We proceed 
in just the same way. We open the cylinder first, then the 
jet tap, and here we go on increasing one gas after the other, 
H always first, till we find that we cannot turn on enough of 
H to redden the light, then the O tap will have to be manipu- 
lated till the best effect is produced. During these steps there 
may be hissing of the gases, but there is no danger, and when 
the light is right the hissing will stop, unless there is some 
defect in the jet. 

Never put on nor take off weights while the gas is burning. 
Never allow any person to lean on, nor in any way to touch a 
gas-bag while the jet burns. 

Always light H first ; always turn down O first. Always 
tie weights on to the pressure-boards, and never use round 
weights. Never use any such things as fenders or boxes of 
glass plates for weight. Bags of heavy sand, made in suitable 
shape and tied on to the pressure-boards answer well. 

Amount of weight, per se, is not the important factor for a 
good light, but proportion of weight to diameter of gas-way 
is the factor that makes or mars. 

If anything goes wrong turn off the O at once, the H as 
soon after as possible. 

A red light means too much H in proportion to O. A 
peculiar lurid blue point of light means too much O for the H. 




CHAPTEK XVI. 
PRACTICE WITH MIXING JETS. 

The use of a mixing jet entails two bags or two cylinders 
with a greater weight in the case of bags, but it enables us to 
get a light certainly twice as brilliant as can be got with a 
blow-through jet, and moreover the area of incandescent lime 
is with the mixing jet considerably smaller than with a blow- 
through, which, as pointed out in an earlier chapter, is an 
advantage of great weight when we desire the best optical 
effect. 

When bags are used it is undeniable that there are possible 
dangers, but the contingency of accident is indeed remote if 
anything like common sense and common care be brought to 
bear on the operations. A mixture of oxygen and hydrogen in 
certain proportions will certainly on ignition explode, and 
that with violence if the gases are confined in any chamber 
offering much resistance. But in olden days the two gases 
used, as the custom, to be mixed in one bag and burned at a 
nipple, yet we are not aware of any accident having happened, 
nor was an accident at all probable so long as there was a 
pressure on the containing bag. JSTow-a-days we put each gas 
in a separate bag, and we put the two bags in the jaws of one 
pair of pressure boards which we weight ; unequal pressure on 
the bags is therefore out of the question, and sucking back of 
one gas into the bag containing the other is seemingly 
physically impossible. But even supposing that the gases do 
get mixed in one bag there is no likelihood, still less any 
certainty of an explosion, provided pressure is kept on the 
bag. All the accidents that have ever come to our notice have 
arisen from setting a light to a mixture of the gases in a bag 
not under pressure, and no reasonable person is likely to do 
such a thing if he is sober, and aware of the effect of igniting 



THE OPTICAL LANTERN. 85 

such a mixture. Even supposing a bag to contain hydrogen 
alone it is a foolish thing to bring a light near it, especially if 
there is no pressure on it. Plydrogen burns on being mixed 
with air, but hydrogen alone is not combustible, nor a supporter 
of combustion. 

A lighted candle thrust into pure hydrogen will be 
extinguished. Even if hydrogen and oxygen are mixed, but one 
of the gases is in large proportion to the other, say nine of H 
to one of O, or nine of O to one of H, there is no explosion 
or ignition, the gases ignite slowly and there is steady com- 
bustion. But two parts of H to one of O will explode with 
violence on ignition. We gather, then, that even if a little of 
one gas gets into a bag containing a considerably larger propor- 
tion of the other gas there is no danger of violent explosion, 
so that in order to produce an explosion we would almost 
require deliberate intention. But each bag should be used for 
one gas alone, and never for the other gas, and each bag should 
be conspicuously marked with " O " or "H," both on the side 
and at the wedge. And if there is any doubt at any time as 
to the contents of a bag, the test should never be made with a 
light placed anywhere near an unweighted bag. Gas cylinders 
in like manner should be conspicuously marked, nreferably by 
being painted in different colors. 

We suppose our two bags to be filled each with its own 
gas, we place the two bags in the pressure boards. First 
we place the H bag in position, taking care to shove the 
tap well through the hole in the pressure boards intended 
for it, we then let down the partition, (see fig. 31) and on 
this we place the O bag. Then we connect the bags with 
the jet tubes by strong rubber tubes, taking care to spring 
the rubbers well up on the taps and jet tubes. Having 
put our weights on the boards we allow a little oxygen to 
escape at the nipple of the jet, and thereafter close the 
O tap. The lime cylinder being on its pin and the surface 
of the lime about one-eighth of an inch from the nipple, 
we light the hydrogen. (If the " cut-off " arrangement, 
(fig. 25) is used, the cut-off cross-piece is turned so that 
both gas-ways are full open.) Now we turn on a little O, 



86 THE OPTICAL LANTEKN. 

when the flame is a brilliant point we raise the H a little, then 
the O again, and so on till we have the best light we can get, 
or until the jet begins to " roar" beyond endurance. (Having 
got the best light, if the cut-ofT is in use, we turn the latter 
completely off, the H will continue to burn gently, and on 
turning the cut-off open again we shall find the light as we left 
it — at its best. Using the cut-off with cylinders alone doe& 
not answer, but with Beard's Regulators on the cylinders, the 
cut-off will work quite well.) For a mixing-jet where two 
bags are used in one pair of pressure boards, having a surface 
of top of 40x30 inches, 112 lbs. will be found a good average 
pressure. In our experience no " composite " lime will stand 
the amount of heat produced by this pressure and a good jet ; 
a " solid " lime will be required, i. <?., one turned from lime- 
stone. 

When we are dealing with multiple lanterns and dissolvers, 
we have, of course, to follow the above course with each jet- 
getting all the jets to burn as nearly as may be with equal 
brilliance, a matter of no slight difficulty at times. But when 
once we have got our jets to work equally well, a good dis- 
solver will ensure the changes being properly made, and if 
our slides are of equal density we shall have equally brilliant 
images on our screen. 

As has been already said, the light should be arranged and 
everything centred and focused before the audience begin to 
enter the hall. This is all the more important when the lan- 
tern is placed among the audience. The cut-off arrangement 
enables us, having once regulated the light, to turn it down 
till we require it for the lecture, and then to get it as we left 
it in an instant. If the " cut-off," or an equivalent, be not 
available, we must at the beginning of the lecture turn up the 
gases again as neatly and as quickly as we can. 

The cautions given in the end of last chapter apply equally 
for the mixing-jet, and ought to be studied beforehand, and 
committed to memory. 



CHAPTER XVII. 
PREPARATIONS FOR LECTURE. 

A person lecturing for the first time cannot reasonably 
expect to meet with such success as he may hope to achieve 
after some practice. This remark holds good for an ordinary 
lecture or public address of whatever kind, but it has much 
greater weight when applied to lectures illustrated by means 
of the lantern, with all its incidental cares. A few remarks 
suggested by experience may therefore not impertinently nor 
inaptly be addressed to the young or the intending lecturer, 
and these remarks naturally range themselves under two 
heads, viz. : points to be noted with regard to the actual lan- 
tern operations ; and points to be attended to by the lecturer 
during his actual occupation of the platform. 

If any fiasco ever takes place in regard to a lantern display, 
it is most likely to happen through some part of the apparatus 
being left behind or not procured. And if anything is for- 
gotten, it is pretty sure to be a small article. One does not, as 
a rule, forget to pack and carry the lantern, nor the pressure 
boards, we are far more likely to forget the key for opening 
the valve of the cylinder. The limes are not seldom left at 
home, and, like the valve-key, are almost impossible to get 
unless the lecture is to be given in a city or large town. We 
propose, therefore, to give, as a sort of memoria technica, a 
full list of all the articles required. This list will be found on 
page 106. 

It is always well to examine the hall where the lecture is to 
be given, before it is actually time to start the erection of the 
screen. Sometimes the hall or the platform is of such a con- 
struction as to prevent the use of our ordinary screen or screen- 
frame, sometimes the arrangements are such that we can 
advantageously dispense with our screen-frame altogether. 



88 THE OPTICAL LANTERN. 

Sometimes there is great difficulty in rinding a suitable site for 
the lantern, and very often, put the lantern where we may, 
there is a gas bracket or some such obstacle between the lan- 
tern and the screen. If we see these matters a day before- 
hand, we can frequently have them remedied in time, if we 
do not see the difficulties till the last moment, we may not 
have time to remove them. Gas brackets in particular have 
to be seen to, we have more than once had to cause the removal 
of a rigid pendant. 

It is by no means necessary to have the lantern on the 
middle line of the hall ; it is often very convenient both for 
lanternist and audience to have the lantern quite at the side of 
the hall ; in such a case, of course, the screen is not erected at 
right angles to the central line of the hall, but at a greater or 
less angle to it. If there is no gallery or other eminence at 
the back of the hall and at a suitable distance from the plat- 
form, this plan of " angling" the screen and putting the lantern 
at one side of the auditorium is not only passable but very 
desirable, for the lantern is not surrounded by the audience 
and none of the audience can be directly behind the lantern, 
and so be dazzled or have the view obstructed by it. We 
strongly recommend the placing of the lantern at one side, 
where rio gallery is available right in front of the screen. 
Anything is better than having the gas-bags or cylinders closely 
surrounded by the audience. 

Undoubtedly the best site for the lantern is the front of a 
gallery straight in front of the screen. Sometimes, however, 
this gallery front is too far distant from the screen for the 
disc we require. Roundly speaking, the diameter of the disc 
should be about one-third of the length from back to front of 
the hall. If we have a hall forty-five feet from gallery to 
platform we will get a fifteen feet disc with a nine-inch lens, 
about a seventeen feet disc with an eight inch lens ; if our 
distance be sixty feet we shall get a twenty feet disc with a nine 
inch lens, a fifteen feet disc with a twelve-inch lens. If we 
have only one projection lens, say of eight inches focal length, 
we will frequently have to go to the body of the hall with our 
lantern. One thing we would try to impress upon our 



THE OPTICAL LANTERN. 89 

reader ; a good small disc is vastly superior to a poor large 
one, and the small bright disc is better seen than the larger 
poorly lighted disc. The foremost of the audience should be 
kept well back from the screen, the larger the disc the further 
back the front row of seats should be placed. In most cases 
the best place for seeing is at about a distance from the screen 
of three times the diameter of the disc, not less certainly than 
two times, but, of course, short-sighted persons will see better 
if they are closer to the screen than these distances. 

"We will suppose the screen to have been erected, the site 
for the lantern chosen, the lantern unpacked and on its table 
or stand in the selected position. In choosing the spot for the 
lantern we must take care that the optical axis of the system 
is perfectly perpendicular to the screen, in other words, that 
the lantern is right opposite the centre of the screen. The 
tilt required for the screen may be judged with fair accuracy 
by eye, but a safer way is to centre the disc on the screen, 
place a slide in the carrier, and tilt the screen until the fore- 
ground and top of the picture are alike in focus. For 
centering the disc we use a blank slide, that is to say, a 
circular mask mounted between two glasses of lantern slide 
size, the centre of the circle being marked on one of the 
glasses with ink or a piece of paper gummed to the glass. 
(To make a good ink-mark on glass lick the glass with the 
tongue, let it dry, then put on the ink. If soot is dusted over 
the ink when dry the mark will be all the better.) It must 
not be forgotten that though the whole of the circular disc 
may be shown on the screen, an oblong (or cushion-shaped) 
picture may overlap the screen, so allowance must be made 
for this. In any case the picture should fall well within the 
limits of the screen, the half-lighted margin of screen round 
the real picture forms a nice mounting for the latter. If it be 
seen that one side of the picture, or of the mask-image on the 
screen is more sharply focused than the other, the screen is 
not at right angles to the optical axis, and the lantern or the 
screen must be shifted. 

The beginner should certainly see to all these matters some 
hours before the show is to begin ; the lantern ought to be in 



90 THE OPTICAL LANTERN. 

position and the lenses focused, so that when the time comes 
to light up there may be no alteration of position required. 
Two or three limes may also at this stage be put to bake in 
an oven, or on a hob, a well-baked lime is a considerable 
advantage. 

Of course, if either or both of the gases is to be made by 
the operator, it should be done a few hours before the display 
is to begin, and we advise the tyro to make plenty of gas. A 
few pence wasted are well repaid by peace of mind, the oldest 
lecturer is not without his qualms as to the gas supply, for one 
never knows what accident may occur. Gas-bags should on 
no account be left without surveillance in an open hall, even 
if the bags be locked ; and if cylinders are used, the owner 
should not surrender charge of the keys. There are always 
clever fellows about who are eager to show off their know- 
ledge, and a cylinder of oxygen yields many beautiful experi- 
ments ! 

Immediately before the final lighting up the front lens 
should be well warmed. The condenser very quickly warms 
when the gas is lighted, but the front lens does not get so 
much heat, so it warms slowly and is apt by " sweating" to 
spoil some of the first pictures. But it may be removed only 
after the image is centred and focused, so that as soon as the 
lens is replaced the image is found to be in statu quo antea. 

During this afternoon visit to the scene of action the 
lecturer should see his platform arranged ; his desk, chair, 
light, signal, water-bottle and glass. It is too late to do these 
things when the audience begins to come in. The lanternist 
may see that his slides are in order, but he must not leave 
them in the hall. Practical jokers are not extinct, and fools 
are numerous. 



CHAPTER XVIII. 
MANAGEMENT OF LANTERN DURING LECTURE. 

Having arranged all our apparatus so as to get a satisfactory 
light, we proceed to centre our disc on the screen, to " register " 
our discs if we propose to dissolve, to focus our light and 
our projection lens. These operations are all to be done by 
experiment and cannot usually be performed in any stated 
order, they depend much on each other. Perhaps the easiest 
plan is to focus the light first. We begin by projecting some 
kind of disc on the screen, a blank slide being in the carrier. 
(The carrier itself is usually centred by removing the projec- 
tion lens, and looking down the nozzle till by eye we get the 
aperture in the carrier central with the nozzle. A very good 
plan is to put a " stop " or make a mark on the carrier when 
centered, so that we may be able at any future time to centre 
it at once.) Probably our disc at first has no particular shape 
and is unevenly lighted ; by pushing and pulling, and moving 
to one side and the other, and by raising and lowering the 
jet we finally get a brilliant round disc evenly lighted, and 
with sharp colorless edges on our screen, and if our lantern is 
properly placed, our disc is in the centre of the screen. As 
already said, if the disc-edge is not equally sharp all round 
the lantern is not "square on" to the screen; if the bottom 
or top of the disc is unsharp the cant or tilt is wrong ; if one 
side is out of focus the angle of screen to lantern is wrong. 
If our pictures are to be of shape other than circular we must 
try a mask of that other shape. We finally remove the blank 
slide, and put in the first slide of the lecture ; this we very 
carefully focus, and we are now ready to begin. 

If the lecture is to last, say, eighty minutes, two limes will 
assuredly be required, three will be better if we have a chance 
of changing more than once. A pair of " pliers " should be 



92 THE OPTICAL LANTEKN. 

at hand to remove the used lime, in fact a pair of plumber's 
pliers is what no lanternist should be without. The hole up 
the middle of the limes should be cleared out before the lec- 
ture, sometimes the lime is not put on its pin without trouble, 
and anything savoring in the smallest degree of a hitch must 
be carefully avoided. The lime must be turned at intervals, 
greater or shorter, according to the force of our jet. A blow- 
through jet requires its lime turned, say, every four minutes ; 
a mixing jet under heavy pressure works best when the lime 
is almost constantly on the move. Clockwork has often been 
used to drive the lime slowly round. Anyhow, a deep-pit on 
the lime must never be allowed. Of course, a soft lime pits 
more readily than a hard one. 

It is a sign of mismanagement when an operator has to 
keep altering his jet-taps ; if the light first attained be of 
proper quality, and the jet properly made, no tampering with 
taps should be needed at all. It is just possible that a few 
minutes after the lighting up a slight alteration may be 
required (for some reason when this happens it is the oxygen 
that requires to be slightly increased, as a rule), but in a 
general way we do not require to alter anything until the 
pressure in the cylinders or bags falls materially lower, which 
is always near the end of a lecture and often does not occur at 
all. But if the light becomes too red or too large in extent, 
we are forced to reduce the hydrogen, which is, perhaps, 
preferable to increasing the oxygen. Unless the jet is in 
some way clamped in its position care is necessary to avoid 
knocking the end of the jet or the rubber tubes, and so uncen- 
tring the light. Moreover, the slides should all slip sweetly 
into the carrier ; we rather object to a carrier into which 
slides are dropped, as the carrier is apt to be knocked out of 
centre unless it is clamped as suggested at page 26. 

The slides must all be in order and convenient to the hand 
of the lanternist, and they should be distinctly marked so that 
the lanternist may know in the semi-obscurity of the lantern- 
vicinity how the slides go into the carrier without having to 
hold them up between his eye and the screen, or near the 
back of the lantern. In England, if there is any standard 



THE OPTICAL LANTERN. 



93 



slide-mark at all, it is this: The slide is laid down as the 
picture actually appears in nature or is intended to appear on 
the screen, and two marks are affixed, one to each tojj corner. 
These two marks go into the carrier next the light and down- 
wards. 

Slides that have passed through the lantern should be kept 
quite apart from those still to pass. 




CHAPTER XIX. 

DEPORTMENT ON THE PLATFORM. 

The young lecturer is sure to be more or less nervous, but in 
different lecturers the nervousness shows itself in different 
ways. One is full of diffidence and tremors, the other is 
assertive and inclined to "swagger." If the diffident one has 
himself superintended all the preparations, knows that his 
apparatus is good, and his gas plentiful, he may keep his mind 
quite at rest, he has done his " level best," and nervousness 
now may spoil the whole. If the assertive one will only 
realize that a forward manner is offensive to his hearers, and 
certain to vitiate his success in their estimation, he may, 
perhaps, subdue for the time his conceit. 

There are some who think that glitter of brass on the lantern 
and gold lace curtains over the screen will make up for any 
shortcomings of the slides or of the lecture, while there are 
others who fancy that disregard of external appearances looks 
business-like and savors of the veteran lecturer. Probably 
both these parties are wrong, and excellence lies midway 
between the two extremes. All apparatus should be in 
thorough working order, and scrupulously clean, and the 
screen is all the better for having the frame-poles concealed by 
a tasteful but simple pair of curtains. On the other hand 
acres of bright brass do not make a good lantern any more 
than a dress suit constitutes a good lecturer. There is a very 
good story told of a lecturer whose lantern was so resplendent 
with bright brass work that he found the audience all sitting 
with their backs to the screen and their faces to the lantern, 
they could not believe that such a very grand instrument was 
not the intended object of their attention. 

There should be a thorough understanding between the 
lecturer and the lanternist, certain signals should be arranged 



THE OPTICAL LANTERN. 95 

for communication between them in case of mistake. For 
example if by some mismanagement a slide comes in the wrong 
order or reversed as to right and left, it is most awkward for the 
lecturer to hold a dialogue with the lanternist. It has already 
been stated that the neatest kind of signal consists of an electric 
communication between speaker and lanternist, the bell at the 
lantern being muffled. The effect is exceedingly good when 
the lantern can be hidden from the audience altogether as in a 
little room at back of the hall, the front of the projection lens 
coming close to a hole made for it, and the lanternist having a 
small window through which he can view the screen and the 
platform. The writer once lectured under the following 
pleasant conditions. A screen 30 feet square, and quite opaque, 
a disc about 28 feet diameter, the screen draped with red 
curtains at each side. The radiant, an electric arc of 12,000 
candles nominal power, worked by an engine in another street. 
The lantern entirely hidden from view, and electric communi- 
cation with the lantern room. The pictures come on the screen 
as if by incantation, for the lecturer designedly concealed his 
" push " from the audience. 

The lecturer ought always to make a few remarks to the 
audience before turning down the gas or other light illuminat- 
ing the hall, and while he is making these introductory remarks 
he should carefully study the faces in various parts of the hall 
in order to learn whether all can hear him well. Ears turned 
to the front, and gaping mouths, are sure signs that the speaker 
is not heard, and he must alter his voice and enunciation 
accordingly. Shouting is never a good way of making oneself 
heard, aloud conversational tone is the utmost amount of force 
likely to be useful. Slow, deliberate enunciation is what is 
wanted. Every syllable must be distinctly pronounced, and 
rapid utterance must never be practised. The nervous lecturer 
is almost certain to speak far too quickly, and consequently is 
very apt to stammer and get mixed in his ideas, so the 
more nervous we are the more carefully must we study slow, 
precise speech. 

In a popular lecture jokes are valuable, in fact nearly 
necessary. But the jokes must not be too stale, a jest familiar 



96 THE OPTICAL LANTERN. 

to everybody is worse than useless. We must try to suit our 
jokes to our audience, jokes that convulse a Scotch audience 
are lost in England on the very same class of people, probably 
a successful " quip " for a California lecture would fall flat in 
Boston. We must not be vexed — or at least we must not show 
vexation — if our jokes miss lire, we must try again with a 
different projectile. The writer places considerable importance 
on a good stock of jests for popular lectures, if we cannot 
compose a sally of wit ourselves we may find what suits the 
purpose in books, and, as we said, provided the jokes are not 
really pre-adamite they are sure to tell, and add to the success 
of the lecture. 

Sometimes a foolish audience is apt to become unruly when 
the gas is turned down. This is almost always the fault of the 
lecture or the lecturer. If the lecture is uninteresting boys 
are sure to lose patience, and indeed who does not sympathize 
with them % It is a fearful trial of patience to sit in the dark 
and see poor slides and hear dismal inanities uttered. If such 
a case should occur, and if any serious interruption to the lec- 
ture took place, we should simply cause the gas to be turned 
up, and decline to proceed unless order should be maintained. 
But the worst thing a lecturer can do in such cases is to lose 
his temper. If he keeps calm, and even benignant, it will be 
a very low audience that will not yield to his good nature. 

In lantern lectures extending beyond forty minutes, there 
should always be an interval during which the hall is illumin- 
ated with the usual lights. This affords a rest to all concerned, 
and allows the lanternist to change his lime and see that all 
his apparatus is in order. But the interval must not be long, 
five minutes is perhaps the longest that will be safe. If the 
audience is put into bad humor by being kept waiting the dan- 
ger of a disturbance is tenfold increased. 

« Unless the lecturer is very glib of speech, and has his subject 
at his finger ends, the lecture ought to be written or printed. 
An extempore lecture when good is a grand success, but when 
poor is apt to be a dismal failure, and the lecturer is set down 
as a conceited fool for attempting it. 

To those unaccustomed to prolonged stretches of public 



THE OPTICAL LANTERN. 97 

speaking, especially when nervousness is felt, a lecture of, say, 
eighty minutes, is a severe tax on the throat. Drinking large 
quantities of water only makes matters worse sometimes, a 
tiny tablet or pellet of chlorate of potash will be found as 
good as anything to take. Borax is sometimes added to the 
potash, but the advantage is very doubtful. Not much liquid 
of any kind should be taken before lecturing, but hot drinks, 
as tea and coffee, are the worst of all things. 

!No one but a very old hand can sing and speak in one 
lecture. 

Ninety minutes is the longest time advisable for the dura- 
tion of a lantern lecture. 




CHAPTEK XX. 

ARRANGEMENTS FOR THE LANTERN IN A LEC- 
TURE-ROOM. 

As one of the chief aims of this book is to simplify and 
increase the use of the optical lantern in lecture-rooms forming 
parts of universities, colleges, schools, and other educational 
and recreative institutions, we may do well to suggest a few of 
the steps that may be taken to render the lantern a permanent 
instalment of the place. In such a case it is indubitably the 
best plan to have an opaque screen, and the best screen is, as 
previously stated, a plastered wall, smooth and matt in surface. 
Almost equally good in all respects, most convenient in many, 
is a strong opaque " faced " screen, such as was described in a 
previous chapter. This may very well roll up like a map, to 
be let down at the teacher's will. If it is convenient to set up 
the lantern straight opposite and at right angles to the screen, 
the latter may be allowed to hang down naturally ; if the lan- 
tern must go below the centre of the screen the roller may be 
brought a few inches out from the wall at top, and the roller 
at foot of the screen may be placed and held close against the 
wall. If the lantern goes more conveniently on a higher level 
than the centre 'of the screen, as will be the case in many lec- 
ture-rooms of the amphitheatre style, then the roller at top 
may be as close to the wall as possible, while the roller at foot 
of the screen may be pulled out and held a few inches from 
the wainscot or lower part of the wall. 

In nearly all cases such as we are treating now a ten feet 
disc will be ample," and a blow-through jet sufficiently 
powerful, indeed, a good oil-lamp will answer for such a disc. 
On the score of convenience the lime-light will probably be 
found preferable, for the requisite illumination can be got up 
more rapidly than with oil, and the effect, especially with the 






THE OPTICAL LANTERN. 99 

ordinary run of slides aud with objects projected through a 
lantern microscope or a polarizer, or a prism, will certainly be 
better with lime than with oil. We recommend a good-sized 
cylinder of oxygen, say forty cubic feet, or else a metal tank 
of the gas, but not a bag if the cylinder or tank can be 
obtained. Of course the tank would not be large enough 
to hold more than six or ten cubic feet. Bags for such a 
purpose would be wasteful even if they were not tampered 
with by the students. While thus recommending the blow- 
through jet, we must say that we have almost entirely given 
up its use, preferring from every point of consideration the 
mixing jet with two cylinders and two regulators. For a 
lecture-room we should suggest one of the open-stage lanterns 
such as Fig. 42 ; such a lantern is useful for every condition 
likely to arise. A part of the outfit ought to be the arrange- 
ment for showing opaque objects. For special illustrations 
such as spectrum analysis special accessories must, of course, 
be added. 

The matter that seems to give teachers the greatest amount 
of perplexity is that of darkening the lecture-room during the 
day time, and even when the gas is lighted. The gas is 
easily arranged by putting within reach of the teacher, the 
lanternist, or an attendant, a bye-pass tap, whereby the gas of 
the room can be lowered " to the blue " without being extin- 
guished. If the electric light be used for the room it can 
easily be switched off, and here it may be said that if 
electricity be used for the illumination of the room or building, 
we should certainly utilize it for the optiGal lantern, even 
were we restricted to an incandescent lamp. The electric 
light has its disadvantages for the lantern, but its perform- 
ance is so good when in good order, and its convenience so 
great at all times that, when available, it should certainly be 
utilized for our purpose. 

For blocking out daylight the simplest and best contri- 
vance is a shutter on the " Louvre " principle. In Britain 
we have a shutter known as Clarke's patent, and it answers 
our purpose admirably. It is made of slips of wood joined 
by strips of strong cloth, and the shutter simply folds up into 



100 THE OPTICAL LANTEKN. 

a coil which occupies, when the shutter is open, a receptacle 
at top or bottom of the window. This shutter, when well 
fitted, completely blocks out light ; it is opened and shut in a 
few seconds. But even this is not necessary to success, for a 
blind made of opaque stuff, fitted in runners, for instance, 
close to the sides of the window, will shut out light suffi- 
ciently for our purpose. Absolute darkness is not essential to 
even a good image on the screen, while a powerful lantern will 
project a useful picture even when the room is only in semi- 
obscurity. We have seen a very successful series of lantern 
illustrations in a room where every person present was visible 
to the lecturer, though only dimly. But it must be under- 
stood that where pictures, and not mere instruction, are the 
object, the room should be as dark as possible ; we only say 
that instruction of a class is possible in a dimly lighted room. 
And if our lantern slides be dense, or our microscopic objects 
thick or heavily stained, then the room must be really dark. 

Throughout this book the writer has had in his mind the 
instructor rather than the entertainer. The optical lantern, 
as a means of imparting instruction to classes, has never occu- 
pied the place it ought to occupy on its merits. In the 
interest of teacher and student alike, we venture to hope that 
the optical lantern will soon take the place it deserves. 




CHAPTER XXI. 
ENLARGING WITH THE OPTICAL LANTERN. 

As those who possess an optical lantern frequently wish to 
use it for enlarging from photographic negatives or positives, 
it may not be out of place for us to give a few remarks on the 
subject. 

If the original negative be of lantern-slide size, or if the 
part we wish to enlarge cover an area of not more than three 
and a quarter inches square, we shall not require any alteration 
of our lantern with four-inch condenser provided the front 
lens racks out far enough. But if we propose to enlarge from 
entire quarter plate negatives, we shall require a larger con- 
denser, and so also for every larger size of negative. To find 
the size of condenser necessary to illuminate properly any size 
of plate which we may wish to enlarge a simple rule is to 
take the diagonal of the plate ; the diameter of the condenser 
must be a little more than the diagonal of the plate. Thus to 
enlarge a " half -plate," 6-|x4§ inches (English size), a con- 
denser of diameter not less than eight and a quarter inches 
should be used. 

We are less tied down in the choice of a projection lens, as 
the focal length is practically immaterial for any such work 
as we are likely to attempt. But lenses sold for the lantern 
only, and not having their visual and actinic foci coincident 
will not answer for enlarging, or will not answer the purpose 
nearly so well as a lens corrected for photography. But any 
photographic lens will be available, if it give rectilinear images 
and if we can rack our lantern front sufficiently to give the 
lens play. 

Manufacturers produce lanterns having the front so made 
as to stretch outwards from the stage to a considerable extent, 
sometimes this motion is attained by bellows, sometimes by 



102 THE OPTICAL LANTERN. 

telescope joints. One probably is as good as the other. The 
reason for this stretch of front lies in the fact that in order to 
get a focused image twice the size of the original, the centre of 
the lens must be one and a half times the focal length of the 
lens distant from the object to be enlarged. Thus, with a six- 
inch lens to get an enlargement twice the area of the original, 
the centre of the lens — usually very near the stop — must be 
nine inches distant from the object to be enlarged. If a copy 
of size equal to that of the original be required, the front 
must rack out so that the centre of the lens can be placed at 
twice the focal length of the lens from the object being copied. 
At the end of this chapter we give a table extracted from the 
almanac of the British Journal of Photography. This table 
shows at a glance the distances from centre of lens to object 
and to receiving surface for various degrees of enlargement or 
reduction. 

The sensitive material on which we project and finally 
develop our enlarged image is, as a rule, the paper known to 
photographers as bromide paper, but it may be a slow gelatine 
plate, or a wet plate or dry collodion plate. The purely 
photographic part of the operations is out of place here ; we 
may refer our reader to the " Processes of Pure Photography,"* 

or any other book of the same series treating of photo- 
graphic operations. 

The substances used for the photography of enlarged images 
are usually very sensitive to actinic light, and bromide paper, 
for instance, will be damaged if any stray light from the 
lantern or from the illumination of the apartment reaches 
it, unless such stray light be of a yellow or red color. If 
there is much light about the apartment, however non-actinic 
the light may be, the difficulty of focusing the image will be 
found great. The best plan is to enclose the entire lantern in 
a box, letting the lens-front alone protrude, and allowing the 
necessary draught to reach the light either by " trapped " 
apertures, or by some arrangement of thick cloth. Reflec- 

*" Processes of Pure Photography," by Burton and Pringle. New 
York: Scovill, 1889. 



THE OPTICAL LANTERN. 



103 



tions, it must be remembered, must be kept from the 
sensitive material as sedulously as direct stray light. 

The paper or other sensitive material is to be placed in front 
of the projection lens, and the receiving surface must be 
perpendicular to the optical axis of the condensing and 
projection systems. If this is not attended to some part of 
the enlarged image will surely be blurred. Paper may be 
tacked with drawing pins to a board or a wall, or it may be 
held in an easel in front of the projection lens. The easel 




Fig. 46. 



may be free on the floor, but it is better when convenient to 
have it either running on a track or sliding in slots or grooves 
along the optical axis of the system. Fig. 46 shows a very 
useful easel made to hold a long roll of bromide paper or cut 
sheets,- as may be desired. 

The accurate focusing of the projected image may be 
adjusted either by racking out and in the projecting lens, o* 



104 THE OPTICAL LANTERN. 

by pushing backwards and forwards the easel. The former is 
perhaps easier, the latter perhaps preferable. If accuracy in 
amount of enlargement is not of vital importance, the former 
method may be adopted. We may project the image and 
focus it either on a white surface, as a sheet of paper, there- 
after replacing the white paper by our sensitive surface ; or 
we may even pin our bromide paper over the plain paper on 
which we focused ; or we may place on the lens a cap of non- 
actinic glass and focus directly on our sensitive surface, 
removing from the lens the non-actinic cap when the focus is 
adjusted. The question of exposure comes under the head 
of photography, so we shall not here discuss it at length. 
The actinic force of the radiant, the intensity ratio of the 
lens, the density of the object to be enlarged, and the sensi- 
tiveness of the receiving surface being all taken as fixed 
terms, the exposure varies directly with the areas of enlarge- 
ment and inversely with the utilized area of the condenser. 



THE OPTICAL LANTEKN. 



105 



TABLE FOR ENLARGEMENTS. 
Copied from the " British Journal Almanac for 1882." 



Focus of Lens. 


Times of Enlargement and Reduction. 


Inches. 
2 


1 

In. 
4 
4 


2 

In. 
6 
3 


3 

In. 

8 
2% 


4 

In. 
10 
2^ 


5 

In. 
12 

2f 


6 

In. 

14 

2i 


7 

In. 
16 

2f 


8 

In. 
18 

2i 


2K 


5 

5 


7K 
3% 


10 
3i 


12& 
3^ 


15 
3 


2H 


20 

2f 


22| 


3 


6 
6 


9 


12 
4 


15 
3M 


18 
3* 


21 
3i 


24 
3f 


27 
3| 


3^ 


7 
7 


10K 

5k' 


14 

4-3 


4M 


21 


24i 
4A 


28 
4 


31£ 

Q15 


4 


8 
8 


12 
6 


16 

5k 


20 
5 


24 

4f 


28 
4f 


32 
4f 


36 

4i 


4K 


9 
9 


13K 

6% 


18 
6 


22^ 
5% 


27 
5f 


3H 
5i 


36 
5* 


40* 
5A 


5 


10 
10 


15 

7K 


20 
6^ 

U 3 


25 

6k 


30 
6 


35 

5| 


40 
5f 


45 

KB 


5K 


11 
11 


16K 

8k 


22 

7^ 


27K 
6^ 


33 
6K 


38J 
fi 6 


44 
6f 


49| 
6A 


6 


12 
12 


18 
9 


24 

8 


30 


36 


42 

7 


48 
6f 


54 
6f 


7 


14 
14 


21 
10^ 


28 

n 


35 

8M 


42 
8f 


49 

8| 


56 

8 


63 


8 


16 
16 


24 
12 


32 

lOf 


40 
10 


48 
9| 


56 


64 
9> 


72 
9 


9 


18 
18 


27 
13^ 


36 
12 


45 

Hk 


54 
lOf 


63 

10i 


72 
lOf 


81 

104 



It is assumed that the photographer knows exactly what the focus of his lens is, and 
that he is able to measure accurately from its optical centre. The use of the table will be 
seen from the following illustration : A photographer has a carte to enlarge to four times 
its size, and the lens he intends employing is one of six inches equivalent focus. He 
must, therefore, look for 4 on the upper horizontal line, and for 6 in the first vertical 
column, and carry his eye to where these two join, which will be at 30 — 7%. The greater 
of these is the distance the sensitive plate must be from the centre of the lens and the 
lesser, the distance of the picture to be copied. To reduce a picture any given number of 
times the same method must be followed, but in this case the greater number will 
represent the distance between the lens and the picture to be copied; the latter, that 
between the lens and the sensitive plate. This explanation will be sufficient for every 
case of enlargement or reduction. 

If the focus of the lens be twelve inches, as this number is not in the column of focal 
lengths, look out for 6 in this column and multiply by 2 ; and so on with any other numbers. 



106 the optical lantern. 

Memoranda for a Lantern Display. 

Lantern : Condenser, projection lenses, support. 

Jet : Limes in box, tubing, needle for nipple, (dissolver and 

tubing). 
(Lamp : Wicks trimmed, oil with camphor, scissors). 
Pressure-boards : Weights. 
Bags : Bag-taps, (back-pressure valves). 
Cylinders : Regulators, key, spanner. 
Screen : Screen-frame, cord, tape, (curtains), a few staples or 

brass hooks with screw. 
Reading-lamp : Signal, manuscript or book, list of slides in 

order — for lecturer. 
Slides in box : List of slides in order — for lanternist. 
Miscellaneous : Gas pliers, hammer and nails, screw driver 

and screws, gimlet and bradawl, instrument to bore out 

lime-hole, file, extra tubing, matches, tape measure. 

Lubricate taps of jet and bags (see that gas way of jet 

is clear). 
For Oxygen Making: Retort, retort neck, tubing, oxygen 

mixture, heating stove and tube if for gas, purifying 

bottles — one or two, metal tubing for same, rubber tubing 

for same, alkali. 
Squeeze all air out of bag, lubricate tap, lock for bag, bag 

marked O. 
For Hydrogen : Generating bottle, scrap zinc, acid, purifier, 

metal tubing, rubber tubing, or, large tubing and metal 

cone, or " adapter " to join gas supply from main of 

building to the H bag, gas pliers and wrench. 
Squeeze air out of bag, bag marked H. 
Note : Never put grease or oil on metal fittings of cylinders, 

nor on regulators, nor gauges. 



FINIS. 



INDEX. 



PAGE 

Angle of Impact on Lime 39 

Angling the Lantern and Screen 88 

Area of Condenser 13 

Arranging the Lantern 91 

Bags, Sizes of 55 

Barium Oxides for Oxygen 

Making 59 

Battery, a one Lens 22 

Beard's Regulator 61 

Benzoline Saturator, Scott's. . . 42 

Best Light, How to Get 40 

Biunial Lantern for Science. . . 77 

Biunial Lanterns 31 

Blow-through System, Disad- 
vantages of 37 

Body of Lantern 23 

Bore of Jet Tubes 40 

Brin's Oxygen Company 59 

Bromide Paper for Enlarging. 102 

Broughton's Ether Tank. 41 

Browning's Spectrum Apparatus 76 

Bull's-eye Condenser 16 

Bye-Pass on Dissolvers 34 

Camphor Added to Paraffin. . . 80 

Canting Lantern Table 70 

Carrier, Self-Centering, Beard's 71 

Carrier, Use of 12 

Carriers, Chadwick and Place. 71 

Carriers for the Slides 70 

Cell to Condenser, To Fit 

Loosely 16 

Chadwick, W. I., His Table.. 20 

Chadwick Carriers 70 

Check Action Lime Turner. ... 45 

Chlorate of Potash 47 

Common Salt in Oxygen Mix- 
ture 48 

Condenser, Area of 13 

Condenser, Construction of 16 

Condenser, Dallmeyer's 17 

Condenser, Double Plano-con- 
vex 16 

Condenser, Focal Length of 

13 et seq. 

Condenser, Functions of 11 

Condenser, Limit of Focal 

Length 13 

Condenser, Mounting in Cell. 16 

Condenser, Plano-convex.. .. 16 

Condenser, Use of 13 



PAGE 

Condenser Elements, Requisites 

for 18 

Condensers, Long Focus 15 

Condensers for Long Focus 

Lenses note, 18 

Curtain Effect 29 

Curtains for Screen 68 

Cut-Off, Newton's " Pringle".. 45 

Cut-Off, Practice with 85 

Cylinders, To be Colored Dif- 
ferently 62 

Cylinders for Gas, Pressure in. 60 

Dallmeyer's Condenser 19 

Dallmeyer's Projection Lens. . 20 

Dangers of Mixed Gases 84 

Darkening Lecture-Rooms. .. . 99 

Deterioration of Gases 57 

Diameter of Disc, Suitable. ... 88 

Disc, Best Size for 88 

Dissolver, the " Star," 34 

Dissolvers, Described 34 

Dissolvers, Functions of 32 

Dissolving Lanterns 29 

Drying Oxygen 51 

Easel for Enlarging 103 

Educational Lecture-Rooms. . . 98 

•'Effects," 29 

Electric Lantern, Newton's. . . 78 

Enlarging 101 

Enlarging, Exposure Required 104 

Enlarging, Size of Condenser. 101 

Enlarging, Table for 105 

Enlarging Easel 103 

Enlarging Lantern, Sketch of.. 101 

Enlarging on Bromide Paper. . 102 

Enlarging Practice, To Focus. 103 

Eth-Oxo System 38 

Ether Tanks 41 

Excelsior Limes 81 

Experiments, Electric, Optical, 

Etc 76 

Faced Screens 65 

Fletcher's Burner 49 

Focal Length of Projection 

Lens 20 

Foci, Conjugate 11 

Focus of Lens and size of 

Room 2l 

Front Lens. The I9 

Gas Bag Taps, Locking 55 



11 



INDEX. 



PAGE 

Gas Bags 51 

Gas Bags, Qualities of 54 

Gas Bags Becoming Stiff. 58 

Gas Brackets 88 

Gas Cylinders, Description of. 60 

Gasoline Saturator, Scott's ... 42 
Hall for Lecture, Examination 

of 87 

Hepworth, T. C., His Method 

of Painting Screens 65 

Hughes, Cylindrical Miniature 26 
Hydrogen, Pure and Carbur- 

etted Compared 53 

Hydrogen, To Prepare. 53 

Impact angle of Gases 39 

Imperfections in Condensers. . 18 

Interchangeable Jets 41 

Ives' Saturator 41 

jet, Blow-through 36 

Jet, Mixing 37 

jet, Newton's Mixing 39 

Jet, Oxy-Calcium 35 

Jet-Taps 41 

Jet-Taps 44 

Jets, Noisy 40 

Keeping O. in Bags 57 

Landscape Single Lens Projec- 
tion 19 

Lantern, Functions of 12 

Lantern, Opening at back of, 

to Cover 27 

Lantern, " The Scovill »*.... v . 24 

Lantern Body, Size of 12 

Lantern Body or Box 23 

Laverne & Co., their Lanterns 24 

Lecture, Duration of 96-97 

Lecture, Preparations for 87 

Lecturing, Manner in. ..... ,. 95 

Lens, Functions of Projection. 11 
Lime, Arrangements for Turn- 
ing 44 

Lime Box, Author's 81 

Lime Discs 46 

Lime-Light, General Descrip- 
tion 35 

Lime-Shield, Wood's 46 

Limes, Baking 90 

Limes, Cracking 46 

Limes, Described 81 

Limes, Excelsior 81 

Limes, Nottingham or Hard.. . 81 

Limes, Soft and Hard 35 

Limes, To Preserve 81 

Manganese Dioxide 47 

Marcy's Sciopticon 23 

Maybridge, His Castanet 

Signal 72 

Mechanical Jet-Stage 46 



PAGE 

Memoranda of Necessaries 106 

Miniature Lantern, Author's. . . 26 

Mixed Gases 84 

Mixing Jets 37 

Mixing Jets, Designs of 37 

Mixing Jets, Practice with 84 

Mounting a Screen on its 

Frame 67 

Multiple Lanterns 29 

Newton, of London, their Lan- 
tern 25 

Newton's, Wright, Projection 

Microscope 75 

Newton's Blow-through Jet 36 

Newton's Mixing Jet 39 

Newton's Scientists' Biunial. . . 77 

Nipples 40 

Noise in Jets, Causes of 40 

Noisy Jets 40 

Noisy Jets, To Cure 40 

Nottingham Limes 81 

Oakley, Lantern made by 26 

Oakley's Cut-Off 46 

Object of the Book 9 

Objections Raised Against 

Lantern 8 

Obstructions in Gas-way 38 

Oil Lamps, Practice with 80 

Oil Lanterns 23 et seq. 

Opaque Objects, front for 24 

Open Stage 26 

Open Stage Lantern 77 

Optical System 10 

Orchard's Oxygen. 60 

Otway's Dissolver 32 

Oxy-Calcium Jet 35 

Oxygen, To Prepare 47 et seq. 

Oxygen Making, Brin's System 59 

Oxygen Making, Practice of. .. 51 

Oxygen Mixture 47 

Oxygen Mixture, To Test 48 

Oxygen Retort 49 

Painting a Screen , 64 

Paraffin, Camphor in 80 

Photographic Lens for Projec- 
tion 19 

Pits and Pitting of Lime 39 

Pits on Lime, Danger of 44 

Place, J., his Mixing Jet 37 

Place's Carriers 71 

Platinum Tip for Nipple 40 

Point of Incandescence 11 

Polarizing Apparatus 76 

Portrait Lens for Projection. . . 19 

Practice with Lime-Light 8i 

Practice with Mixing Jets 84 

Precautions with Lime-Light. 83 

Preparation of Oxygen 47 






INDEX. 



• • * 

111 



PAGE 

Preservation of Limes 82 

Pressure and Bore 40 

Pressure Boards 55 

Pressure Gauge 62 

Pringle Cut-Off, The. 45 

Projection Lens 11 

Projection Lens, Back Combi- 
nation of 21 

Projection Lens, Dallmeyer's. 20 
Projection Lens, Focal Length 

of 20 

Projection Lens, Taylor & 

Hobson's 20 

Projection Lens, The 19 

Projection Microscope 74-75 

Pumphrey's Mechanical Stage. 46 

Purifying Bottle for Gases .... 50 

Purifying Oxygen 50 

Radiant, Position of 13 

Radiant, The 11 

Reading Lamp 72 

Refulgent Oil Lamp, Newton 24 

Regulator, Beard's 61 

Requisites for Projection Lens. 19 

Retort, Oxygen 49 

Retort, Safety Valve for 49 

Retort, To Clean Out 52 

Rolling Up Screens 88 

Safety-Chamber for Saturators. 43 

Safety-Chambers 38 

Safety Jet 36 

Safety Valve, Chadwick's 38 

Saturator 41 

Scientists' Biunial, Newton.. . . 77 

Sciopticon, Marcy's 23 

Sciopticon Dissolver 30 

Scott's Saturator 42 

" Scovill " Lantern, the 24 

Screen, Functions of 11 

Screen, Permanent 64 

Screen, Pigments for 64 

Screen, Portable 65 

Screen, Tilting . 21 

Screen, To Mount a. . . , 67 

Screen Elevators 66 

Screen Frames 66 

Screens, Faced with Paper. ... 65 



PAGE 

Screens, Good and Bad 64 

Sharpness, Depends on 11 

Shutters for Window, Clarke's 99 

Signals 72 

Signals to be Arranged 94 

Site for Lantern in Hall 88 

Sketching in the Lantern 78 

Slide, Shapes of 11 

Slide, Size of 10 

Slide, The 10 

Slide Boxes 73 

Slide Carriers 70 

Smith, George, Sciopticon Co. 30 

Spectrum Projection, Browning 76 

Stand for Lantern 69 

Standard Mark for Slides 91 

Storage of Gases 54 

Sweating of Condenser 16 

Sweating of Glasses 90 

Table For Enlargements 105 

Table for Cylinders 63 

Tank for Stage 76 

Tanks for Gases 57 

Taps of Jets 41 

Taylor, J. Traill, His Conden- 

ers 16 

Throat and Voice, The 97 

Tilt of Screen, To Judge 89 

To Calculate Pressure on Bags 56 
To Find Distance, Disc, 

Focus, etc 20 

Tone of Voice 95 

Triple Condenser, Taylor's.... 17 

Triunial Lanterns 30 

Triunial Lanterns 32 

Tubing, Qualities and Sizes of 72 

Turning the Lime 44 

Unruly Audience 96 

Varieties of Radiant 11 

Weights for Gas Bags 56-57 

Woodbury, W. B., His Experi- 
ments 76 

Wright, Lewis, His Jet 39 

Wright's Projection Micro- 
scope 75 

Writing on Glass 89 

Zinc White for Screens 65 




mnnm * 



/a